Fixed Nullable value-type fields in Views in C# module causing excessive logging (#3949)

# Description of Changes

This PR fixes a C# codegen performance/behavior issue triggered by views
that include nullable value-type fields (e.g. `DbVector2?`), as reported
in #3914.

* Updated the C# BSATN code generator to emit non-boxing equality for
`Nullable<T>` members by using `System.Nullable.Equals(a, b)` rather
than `a.Equals(b)` (which can box and cause excessive host logging/work
in view diff/equality paths).

This causes code generation to change from something like:
``` csharp
// From:
var ___eqNullableIntField = this.NullableIntField.Equals(that.NullableIntField);
// To:
var ___eqNullableIntField = System.Nullable.Equals(
    this.NullableIntField,
    that.NullableIntField
);
```
* Added a regression scenario to the C# regression-test module that
exercises the problematic pattern:
  * A table containing a nullable struct field (`DbVector2? Pos`).
  * A public view that returns rows containing that nullable field.
* A reducer to mutate the nullable field from `some` → `none` → `some`
to force view re-evaluation and diffing.
* Updated the regression-test client to:
  * Subscribe to the new view.
* Validate that the view evaluates successfully and contains the
expected rows.
  * Call the reducer and validate view state after updates.
  * Fail the test if view evaluation/diffing produces errors.

# API and ABI breaking changes

None.
* No changes to public SpacetimeDB schema or wire format semantics.
* Changes are limited to generated equality code and regression tests.

# Expected complexity level and risk

2 - Low
* The codegen change is small and localized (special-casing
`System.Nullable<T>` equality generation).
* Risk is primarily around subtle behavior differences in equality for
nullable value types; however, `System.Nullable.Equals` matches the
expected semantics and avoids boxing.
* Regression tests specifically cover the nullable-struct-in-view
scenario to guard against regressions.

# Testing

<!-- Describe any testing you've done, and any testing you'd like your
reviewers to do,
so that you're confident that all the changes work as expected! -->

- [X] Ran the C# regression test suite via `run-regression-tests.sh` and
confirmed no errors.

Author: rekhoff

crates/bindings-csharp/BSATN.Codegen/Type.cs

@@ -95,6 +95,7 @@ public static TypeUse Parse(ISymbol member, ITypeSymbol typeSymbol, DiagReporter
                     typeInfo,
                     Parse(member, named.TypeArguments[0], diag)
                 ),
+                "System.Nullable<T>" => new NullableUse(type, typeInfo),
                 _ => named.IsValueType
                     ? (
                         named.TypeKind == Microsoft.CodeAnalysis.TypeKind.Enum
@@ -182,6 +183,24 @@ public override string GetHashCodeStatement(string inVar, string outVar, int lev
         $"var {outVar} = {inVar}.GetHashCode();";
 }
 
+/// <summary>
+/// A use of a nullable value type (e.g. <c>int?</c>, <c>MyStruct?</c>).
+/// </summary>
+/// <param name="Type"></param>
+/// <param name="TypeInfo"></param>
+public record NullableUse(string Type, string TypeInfo) : TypeUse(Type, TypeInfo)
+{
+    public override string EqualsStatement(
+        string inVar1,
+        string inVar2,
+        string outVar,
+        int level = 0
+    ) => $"var {outVar} = System.Nullable.Equals({inVar1}, {inVar2});";
+
+    public override string GetHashCodeStatement(string inVar, string outVar, int level = 0) =>
+        $"var {outVar} = {inVar}.GetHashCode();";
+}
+
 /// <summary>
 /// A use of a reference type.
 /// </summary>

crates/bindings-csharp/Codegen.Tests/fixtures/client/snapshots/Type#PublicTable.verified.cs

@@ -283,7 +283,10 @@ public bool Equals(PublicTable that)
                 }
             }
         }
-        var ___eqNullableValueField = this.NullableValueField.Equals(that.NullableValueField);
+        var ___eqNullableValueField = System.Nullable.Equals(
+            this.NullableValueField,
+            that.NullableValueField
+        );
         var ___eqNullableReferenceField =
             this.NullableReferenceField == null
                 ? that.NullableReferenceField == null

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/Module#TestDefaultFieldValues.verified.cs

@@ -307,7 +307,10 @@ public bool Equals(PublicTable that)
                 }
             }
         }
-        var ___eqNullableValueField = this.NullableValueField.Equals(that.NullableValueField);
+        var ___eqNullableValueField = System.Nullable.Equals(
+            this.NullableValueField,
+            that.NullableValueField
+        );
         var ___eqNullableReferenceField =
             this.NullableReferenceField == null
                 ? that.NullableReferenceField == null

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/Module#TestUniqueNotEquatable.verified.cs

@@ -97,7 +97,10 @@ public bool Equals(CustomClass? that)
             this.StringField == null
                 ? that.StringField == null
                 : this.StringField.Equals(that.StringField);
-        var ___eqNullableIntField = this.NullableIntField.Equals(that.NullableIntField);
+        var ___eqNullableIntField = System.Nullable.Equals(
+            this.NullableIntField,
+            that.NullableIntField
+        );
         var ___eqNullableStringField =
             this.NullableStringField == null
                 ? that.NullableStringField == null

crates/bindings-csharp/Codegen.Tests/fixtures/server/snapshots/Module#PublicTable.verified.cs

@@ -147,7 +147,10 @@ public bool Equals(CustomNestedClass? that)
                 ? that.NestedNullableClass == null
                 : this.NestedNullableClass.Equals(that.NestedNullableClass);
         var ___eqNestedEnum = this.NestedEnum == that.NestedEnum;
-        var ___eqNestedNullableEnum = this.NestedNullableEnum.Equals(that.NestedNullableEnum);
+        var ___eqNestedNullableEnum = System.Nullable.Equals(
+            this.NestedNullableEnum,
+            that.NestedNullableEnum
+        );
         var ___eqNestedTaggedEnum =
             this.NestedTaggedEnum == null
                 ? that.NestedTaggedEnum == null

crates/bindings-csharp/Codegen.Tests/fixtures/server/snapshots/Type#CustomClass.verified.cs

@@ -97,7 +97,10 @@ public bool Equals(CustomRecord? that)
             this.StringField == null
                 ? that.StringField == null
                 : this.StringField.Equals(that.StringField);
-        var ___eqNullableIntField = this.NullableIntField.Equals(that.NullableIntField);
+        var ___eqNullableIntField = System.Nullable.Equals(
+            this.NullableIntField,
+            that.NullableIntField
+        );
         var ___eqNullableStringField =
             this.NullableStringField == null
                 ? that.NullableStringField == null

crates/bindings-csharp/Codegen.Tests/fixtures/server/snapshots/Type#CustomNestedClass.verified.cs

@@ -94,7 +94,10 @@ public bool Equals(CustomStruct that)
             this.StringField == null
                 ? that.StringField == null
                 : this.StringField.Equals(that.StringField);
-        var ___eqNullableIntField = this.NullableIntField.Equals(that.NullableIntField);
+        var ___eqNullableIntField = System.Nullable.Equals(
+            this.NullableIntField,
+            that.NullableIntField
+        );
         var ___eqNullableStringField =
             this.NullableStringField == null
                 ? that.NullableStringField == null

crates/bindings-csharp/Codegen.Tests/fixtures/server/snapshots/Type#CustomRecord.verified.cs

@@ -1,4 +1,4 @@
-/// Regression tests run with a live server.
+/// Regression tests run with a live server.
 /// To run these, run a local SpacetimeDB via `spacetime start`,
 /// then in a separate terminal run `tools~/run-regression-tests.sh PATH_TO_SPACETIMEDB_REPO_CHECKOUT`.
 /// This is done on CI in .github/workflows/test.yml.
@@ -65,6 +65,7 @@ void OnConnected(DbConnection conn, Identity identity, string authToken)
             "SELECT * FROM players_at_level_one",
             "SELECT * FROM my_table",
             "SELECT * FROM Admins",
+            "SELECT * FROM nullable_vec_view",
         ]);
 
     // If testing against Rust, the indexed parameter will need to be changed to: ulong indexed
@@ -87,6 +88,21 @@ void OnConnected(DbConnection conn, Identity identity, string authToken)
         Log.Info($"Got OnUnhandledReducerError: {exception}");
         waiting--;
         ValidateBTreeIndexes(ctx);
+        ValidateNullableVecView(ctx);
+    };
+
+    conn.Reducers.OnSetNullableVec += (ReducerEventContext ctx, uint id, bool hasPos, int x, int y) =>
+    {
+        Log.Info("Got SetNullableVec callback");
+        waiting--;
+        if (id == 1)
+        {
+            ValidateNullableVecView(ctx, hasPos, x, y);
+        }
+        else
+        {
+            ValidateNullableVecView(ctx);
+        }
     };
 }
 
@@ -113,6 +129,44 @@ void ValidateBTreeIndexes(IRemoteDbContext conn)
     Log.Debug("   Indexes are good.");
 }
 
+void ValidateNullableVecView(
+    IRemoteDbContext conn,
+    bool? expectedHasPos = null,
+    int expectedX = 0,
+    int expectedY = 0
+)
+{
+    Log.Debug("Checking nullable vec view...");
+    Debug.Assert(conn.Db.NullableVecView != null, "conn.Db.NullableVecView != null");
+    Debug.Assert(
+        conn.Db.NullableVecView.Count >= 2,
+        $"conn.Db.NullableVecView.Count = {conn.Db.NullableVecView.Count}"
+    );
+
+    var rows = conn.Db.NullableVecView.Iter().ToList();
+    Debug.Assert(rows.Any(r => r.Id == 1));
+    Debug.Assert(rows.Any(r => r.Id == 2));
+
+    var remoteRows = conn.Db.NullableVecView.RemoteQuery("WHERE Id = 1").Result;
+    Debug.Assert(remoteRows != null && remoteRows.Length == 1);
+    Debug.Assert(remoteRows[0].Id == 1);
+
+    if (expectedHasPos is bool hasPos)
+    {
+        var row1 = rows.First(r => r.Id == 1);
+        if (!hasPos)
+        {
+            Debug.Assert(row1.Pos == null, "Expected NullableVecView row 1 Pos == null");
+        }
+        else
+        {
+            Debug.Assert(row1.Pos != null, "Expected NullableVecView row 1 Pos != null");
+            Debug.Assert(row1.Pos.X == expectedX, $"Expected row1.Pos.X == {expectedX}, got {row1.Pos.X}");
+            Debug.Assert(row1.Pos.Y == expectedY, $"Expected row1.Pos.Y == {expectedY}, got {row1.Pos.Y}");
+        }
+    }
+}
+
 void OnSubscriptionApplied(SubscriptionEventContext context)
 {
     applied = true;
@@ -160,6 +214,8 @@ void OnSubscriptionApplied(SubscriptionEventContext context)
     Debug.Assert(context.Db.Admins != null, "context.Db.Admins != null");
     Debug.Assert(context.Db.Admins.Count > 0, $"context.Db.Admins.Count = {context.Db.Admins.Count}");
 
+    ValidateNullableVecView(context, expectedHasPos: true, expectedX: 1, expectedY: 2);
+
     Log.Debug("Calling Iter on View");
     var viewIterRows = context.Db.MyPlayer.Iter();
     var expectedPlayer = new Player
@@ -230,6 +286,14 @@ void OnSubscriptionApplied(SubscriptionEventContext context)
     Debug.Assert(anonViewRemoteQueryRows != null && anonViewRemoteQueryRows.Result.Length > 0);
     Debug.Assert(anonViewRemoteQueryRows.Result.First().Equals(expectedPlayerAndLevel));
 
+    Log.Debug("Calling SetNullableVec (null)");
+    waiting++;
+    context.Reducers.SetNullableVec(1, false, 0, 0);
+
+    Log.Debug("Calling SetNullableVec (some)");
+    waiting++;
+    context.Reducers.SetNullableVec(1, true, 7, 8);
+
     // Procedures tests
     Log.Debug("Calling InsertWithTxRollback");
     waiting++;