Reinstate LINQ join convenience overloads and fix the build break (#126649)

> [!NOTE]
> This PR description was generated with Copilot.

Reinstates the LINQ convenience overloads from #121998 and #121999 that
were reverted in #126624, while also fixing the build break that caused
the revert.

## Summary
- preserves the original API/implementation commits by cherry-picking
them onto this branch
- fixes the `System.Linq.AsyncEnumerable` test build break by making the
affected async selectorless overload calls explicit where inference was
insufficient during the multi-target build

## Validation
- `build.cmd clr+libs -rc release`
- `.\dotnet.cmd build
.\src\libraries\System.Linq.AsyncEnumerable\tests\System.Linq.AsyncEnumerable.Tests.csproj
/t:Test --no-restore`
- `.\dotnet.cmd build
.\src\libraries\System.Linq\tests\System.Linq.Tests.csproj /t:Test
--no-restore`
- `.\dotnet.cmd build
.\src\libraries\System.Linq.Queryable\tests\System.Linq.Queryable.Tests.csproj
/t:Test --no-restore`

---------

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Shay Rojansky <roji@roji.org>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: roji <1862641+roji@users.noreply.github.com>

Author: 4 people

src/libraries/System.Linq.AsyncEnumerable/ref/System.Linq.AsyncEnumerable.cs

@@ -73,6 +73,8 @@ public static partial class AsyncEnumerable
         public static System.Collections.Generic.IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, TKey> keySelector, System.Func<TKey, System.Collections.Generic.IEnumerable<TSource>, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> keySelector, System.Func<TSource, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TElement>> elementSelector, System.Func<TKey, System.Collections.Generic.IEnumerable<TElement>, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, TKey> keySelector, System.Func<TSource, TElement> elementSelector, System.Func<TKey, System.Collections.Generic.IEnumerable<TElement>, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<System.Linq.IGrouping<TOuter, TInner>> GroupJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<System.Linq.IGrouping<TOuter, TInner>> GroupJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Func<TOuter, System.Collections.Generic.IEnumerable<TInner>, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, System.Collections.Generic.IEnumerable<TInner>, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<(int Index, TSource Item)> Index<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source) { throw null; }
@@ -81,6 +83,8 @@ public static partial class AsyncEnumerable
         public static System.Collections.Generic.IAsyncEnumerable<TSource> Intersect<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> first, System.Collections.Generic.IAsyncEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Func<TOuter, TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter Outer, TInner Inner)> Join<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Threading.Tasks.ValueTask<TSource> LastAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, bool> predicate, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Threading.Tasks.ValueTask<TSource> LastAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<bool>> predicate, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Threading.Tasks.ValueTask<TSource> LastAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
@@ -92,6 +96,8 @@ public static partial class AsyncEnumerable
         public static System.Threading.Tasks.ValueTask<TSource> LastOrDefaultAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, TSource defaultValue, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Func<TOuter, TInner?, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter, TInner?, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter Outer, TInner? Inner)> LeftJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Threading.Tasks.ValueTask<long> LongCountAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, bool> predicate, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Threading.Tasks.ValueTask<long> LongCountAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<bool>> predicate, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
         public static System.Threading.Tasks.ValueTask<long> LongCountAsync<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) { throw null; }
@@ -114,6 +120,8 @@ public static partial class AsyncEnumerable
         public static System.Collections.Generic.IAsyncEnumerable<TSource> Reverse<TSource>(this System.Collections.Generic.IAsyncEnumerable<TSource> source) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> RightJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Func<TOuter?, TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TResult>> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> RightJoin<TOuter, TInner, TKey, TResult>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Func<TOuter?, TInner, TResult> resultSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> outerKeySelector, System.Func<TInner, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<TKey>> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
+        public static System.Collections.Generic.IAsyncEnumerable<(TOuter? Outer, TInner Inner)> RightJoin<TOuter, TInner, TKey>(this System.Collections.Generic.IAsyncEnumerable<TOuter> outer, System.Collections.Generic.IAsyncEnumerable<TInner> inner, System.Func<TOuter, TKey> outerKeySelector, System.Func<TInner, TKey> innerKeySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, System.Collections.Generic.IAsyncEnumerable<TResult>> selector) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, System.Collections.Generic.IEnumerable<TResult>> selector) { throw null; }
         public static System.Collections.Generic.IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this System.Collections.Generic.IAsyncEnumerable<TSource> source, System.Func<TSource, int, System.Collections.Generic.IAsyncEnumerable<TResult>> selector) { throw null; }