perf: restrict AbstractTestClassWithDataSourcesAnalyzer to source assembly

[thomhurst]

Summary

• Restrict type scanning to source assembly only: Changed GetAllNamedTypes(context.Compilation.GlobalNamespace) to GetAllNamedTypes(context.Compilation.Assembly.GlobalNamespace). Previously the analyzer recursively walked every type in the entire compilation (including all referenced assemblies) for each abstract test class analyzed. Now it only scans types defined in the source assembly, which is all that's needed since subclasses must be in the user's code.
• Remove redundant IsInSource filter: Since Assembly.GlobalNamespace only contains types from the source assembly, the subsequent type.Locations.Any(l => l.IsInSource) check was redundant and has been removed.
• Replace fragile string-based data source detection with interface check: Removed the typeName.Contains("DataSource") || typeName == "ArgumentsAttribute" string check that walked the type hierarchy. The IDataSourceAttribute interface check is comprehensive and authoritative -- all data source attributes (ArgumentsAttribute, MethodDataSourceAttribute, ClassDataSourceAttribute, etc.) implement IDataSourceAttribute either directly or through ITypedDataSourceAttribute<T> / IAsyncUntypedDataSourceGeneratorAttribute.

Closes #4864

Test plan

• All 12 existing AbstractTestClassWithDataSourcesAnalyzerTests pass on both .NET 8 and .NET 9
• dotnet build TUnit.Analyzers/TUnit.Analyzers.csproj succeeds with no errors

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

Summary

This is a well-targeted performance improvement to AbstractTestClassWithDataSourcesAnalyzer. All three changes are correct:

1. Restricting to context.Compilation.Assembly.GlobalNamespace — Concrete subclasses with [InheritsTests] can only exist in the user's source assembly, never in referenced libraries. Scanning the full compilation was pure wasted work.

2. Removing the IsInSource filter — This guard was only needed because the previous scan included metadata types from referenced assemblies. With Assembly.GlobalNamespace, every returned type is guaranteed to be in source, making the check redundant.

3. Replacing string-based detection with the interface check — All TUnit data source attributes (ArgumentsAttribute, MethodDataSourceAttribute, ClassDataSourceAttribute, MatrixDataSourceAttribute, CombinedDataSourcesAttribute, DataSourceGeneratorAttribute<T>, FsCheckPropertyAttribute, etc.) implement IDataSourceAttribute either directly or transitively. The AllInterfaces Roslyn API handles transitive interface resolution, so manually walking the base-type chain is unnecessary. The interface check is both more authoritative and more efficient.