Extract diagnostic reporting out of COM class generator

[DoctorKrolic]

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[Copilot]

Pull request overview

This PR refactors COM class generation by moving diagnostic reporting responsibilities out of ComClassGenerator and into a dedicated Roslyn analyzer, keeping the generator focused on code emission.

Changes:

• Introduced ComClassGeneratorDiagnosticsAnalyzer to report COM class diagnostics via analyzer infrastructure.
• Updated COM class diagnostics tests to validate analyzer-reported diagnostics rather than generator-reported diagnostics.
• Simplified generator plumbing by removing shared “diagnostic splitting/reporting” helper extensions and adjusting ComClassInfo to return nullable results instead of DiagnosticOr<T>.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/ComClassGeneratorOutputShape.cs	Small assertion refactor for type argument validation.
src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Unit.Tests/ComClassGeneratorDiagnostics.cs	Switches verification to run the new diagnostics analyzer and expands diagnostic test cases.
src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/IncrementalGeneratorInitializationContextExtensions.cs	Removes diagnostics registration helpers no longer used after diagnostics extraction.
src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/DiagnosticOr.cs	Removes DiagnosticOr<T> incremental-provider helper extensions now unused.
src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComClassInfo.cs	Converts from DiagnosticOr<ComClassInfo> to ComClassInfo? and defers diagnostics to the analyzer.
src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComClassGenerator.cs	Updates generator pipeline to use TryGetFrom and filter null results.
src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/Analyzers/ComClassGeneratorDiagnosticsAnalyzer.cs	New analyzer that reports COM class diagnostics and suppresses warnings when errors are present.

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComClassGenerator.cs:40

• After filtering with .Where(info => info is not null), info is still a nullable reference type (ComClassInfo?) for subsequent stages. With Nullable=enable and TreatWarningsAsErrors=true in this repo, passing info to ItemAndSyntaxes<ComClassInfo> and dereferencing it will produce nullability warnings that fail the build. Consider unwrapping to a non-nullable provider (e.g., add a .Select(info => info!) after the Where, or use a helper that returns IncrementalValuesProvider<ComClassInfo>).

                .Where(static info => info is not null);

            var classInfoType = attributedClasses
                .Select(static (info, ct) => new ItemAndSyntaxes<ComClassInfo>(info,
                [
                    GenerateClassInfoType(info.ImplementedInterfacesNames.Array).NormalizeWhitespace(),
                    GenerateClassInfoAttributeOnUserType(info.ContainingSyntaxContext, info.ClassSyntax).NormalizeWhitespace()
                ]));

[jtschuster]

Just a couple nits from me

[jtschuster]

Thank you!