ConfigureAwait(false) gaps in SelectMany/SelectManyAsync iterators

[thomhurst]

Found in the pre-4.0 audit (follow-up to #360/#361). Hygiene, not a blocker.

The library is otherwise consistent about ConfigureAwait(false), but the SelectMany/SelectManyAsync iterator families have gaps:

• AsyncEnumerableExtensions: the await foreach (... .WithCancellation(ct)) loops (both outer and inner) omit .ConfigureAwait(false).
• EnumerableExtensions.SelectManyAsync (IAsyncEnumerable-returning selector overloads): same pattern on the inner await foreach.

Impact: on a sync-context caller (classic ASP.NET, UI frameworks), continuations bounce through the captured context per item — a throughput hit and inconsistent with the rest of the library.

Fix: mechanical — add .ConfigureAwait(false) to each WithCancellation(...) enumeration in those methods. No behavior contract change.

[thomhurst]

Resolved by #370 (merged as commit 9c4c40f). All WithCancellation(...) enumerations in the SelectMany/SelectManyAsync iterator families now use ConfigureAwait(false). Verified on current main; full dotnet test passes 1,716/1,716 tests across net8.0, net9.0, and net10.0.