cleanup: remove unused types

[TimothyMakkison]

Repeat of #4575 without the git issues

[thomhurst]

Summary

Removes unused types and methods from TUnit.Core.SourceGenerator.

Critical Issues

None found ✅

All removed code is verified unused:

• Model files: DiagnosticContext, PropertyDataModel, GenericTestRegistration, PropertyInjectionContext, StaticClassDataSourceInjectorModel, TestDefinitionContext, TestHookCollectionDataModel, TypeWithDataSourceProperties - all only referenced in their own definitions
• Methods in TestMetadataGenerator.cs:
  • GenerateSpecificGenericInstantiation - never called
  • GenerateAotFriendlyInvokers - never called
  • ProcessGenerateGenericTestAttribute - never called
  • IsAsyncMethod - duplicate, TestMethodAnalyzer has its own implementation
  • ReturnsValueTask - never called
  • GetDefaultValueString - never called
  • IsGenericTypeParameter - never called
  • ValidateTypeConstraints(INamedTypeSymbol, ...) - overload never called (IMethodSymbol overload is still used)
• Parameter: combinationGuid in GenerateGenericTestWithConcreteTypes - never used in method body

TUnit Rules Check

✅ No dual-mode changes (source-gen/reflection)
✅ No source generator output changes (internal cleanup only)
✅ No public API changes
✅ No snapshot test updates needed
✅ No performance impact
✅ No VSTest usage

Verdict

✅ APPROVE - Clean removal of dead code with no functional impact