[C#] Removes Query<TRow> and .Build() in favor of IQuery<TRow>

[rekhoff]

This fixes an issue with the C# implementation of Query Builder requiring Build() to return a query, and the return type being Query<TRow> rather than IQuery<TRow>.

Description of Changes

1. Runtime/query-builder

• Removed the concrete Query<TRow> carrier type and every .Build(). Query builder shapes now expose only IQuery<TRow> plus ToSql().
• Ensured all builder entry points (tables, joins, filters) continue to return IQuery<TRow>.

2. Source generator + bindings

• Updated ViewDeclaration analysis to treat any return type implementing SpacetimeDB.IQuery<TRow> as a SQL view.
• Dispatcher generation now emits ViewResultHeader.RawSql(returnValue.ToSql()) . This eliminates a Query<TRow> special-case.

3. Tests, fixtures, regression module

• Converted the C# query-builder unit tests, codegen fixtures, and regression-test server views to call ToSql()/return IQuery<TRow>.
• Added coverage proving RightSemiJoin (and friends) still satisfy IQuery<TRow>.

4. CLI templates & generated bindings

• Regenerated/edited C# template bindings so SubscriptionBuilder.AddQuery accepts Func<QueryBuilder, IQuery<TRow>> and captures SQL via ToSql().

API and ABI breaking changes

While technically API breaking, this actually brings the API closer to the intended design.

• Query<TRow> has been removed from the public surface area; any previous references (including .Build() and .Sql) must be replaced with the builder instance itself plus .ToSql().
• View methods must now return an IQuery<TRow> (or any custom type implementing it) when producing SQL for the host.
• Generated C# client bindings now expect typed subscription callbacks to produce IQuery<TRow>, aligning the client SDK with the new runtime contract.

Expected complexity level and risk

3 - Medium: Touches runtime, codegen, fixtures, and templates. Risk is mitigated by parity with Rust semantics and comprehensive test updates, but downstream modules must recompile to adopt the new interface.

Testing

• Built CLI and ran regression tests locally with removed .Build()
• Ran dotnet test .\sdks\csharp\tests~\tests.csproj -c Release with all tests passing
• Ran dotnet test .\crates\bindings-csharp\Codegen.Tests\Codegen.Tests.csproj -c Release with all tests passing