fix: Nested class test filtering in VS and Test Explorer

[thomhurst]

Summary

• Fixes nested class tests being invisible to VS Test Explorer's "Run Tests" command and appearing as flat siblings without outer class context
• Tree node filter paths now use full OuterClass+InnerClass hierarchy instead of just InnerClass
• ClassMetadata.Parent is now recursively populated in both source-gen and reflection modes
• Filter matching (CouldDescriptorMatch, CouldTypeMatch) handles both full nested paths and partial names

Closes #4144

Test plan

• New NestedClassFilteringTests engine tests verify:
  • /*/*/NestedTestClassTests+NestedClass/* finds 1 nested test
  • /*/*/NestedTestClassTests+NestedClass/Inner finds specific nested test
  • /*/*/NestedTestClassTests/Outer still finds outer class test
• All 448 source generator snapshot tests pass
• ExpectedStateTests pass (comprehensive pass/fail validation across all modes)
• UidFilterMatchingTests pass (28 tests, updated to use new nested format)
• OverlappingClassNameFilterTests pass (no regression in substring matching fix)
• Both source-gen and reflection modes verified

🤖 Generated with Claude Code

[claude]

Code Review

I found one issue that needs to be addressed:

Missing Snapshot Test for Nested Class Test

File: TUnit.TestProject/NestedTestClassTests.cs (new file)

According to CLAUDE.md Rule #2, changes to source generator output require running snapshot tests and committing .verified.txt files.

Since this PR adds a new test file with nested classes AND modifies the source generator to handle nested classes differently (recursive parent generation, + separator naming), a snapshot test is required following the established pattern.

Required files missing:

• TUnit.Core.SourceGenerator.Tests/NestedTestClassTests.cs - snapshot test
• TUnit.Core.SourceGenerator.Tests/NestedTestClassTests.Test.verified.txt - verified output

Why this matters:
The source generator now produces different output for nested classes:

1. Recursive ClassMetadata.Parent chain
2. Test descriptors using OuterClass+InnerClass format instead of just InnerClass

These changes should be captured in a snapshot test to prevent regressions.

Reference pattern:
See existing snapshot tests like TUnit.Core.SourceGenerator.Tests/StaticClassTests.cs for how to structure the snapshot test for this new test class.

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Otherwise: The implementation looks solid. The dual-mode support (source-gen + reflection) is properly maintained, the recursive parent chain logic is correct, and the nested class naming convention using + follows .NET's standard Type.FullName format. The filter matching handles all the relevant cases (exact match, partial inner name, outer class prefix, generics). No bugs detected.