Preserve C# test taxonomy across multiline attributes

[Widthdom]

Summary

• Replace the backward physical-line check with a bounded lexical attribute-block scanner integrated into the existing C# masking pass.
• Preserve xUnit, MSTest, and NUnit test.method taxonomy across multiline attributes while rejecting comments/literals, ignored targets, generic attribute type arguments, collection expressions, fields, indexers, constructors, and member-boundary leaks.
• Advance the C# extractor contract to 17 and add direct plus persisted CLI coverage, including regression coverage for Test method kind not distinguished from functions #1961.

Validation

• dotnet build CodeIndex.sln --no-restore (0 warnings, 0 errors)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue5192|FullyQualifiedName~Extract_CSharp_ClassifiesTestMethodsFromAttributes" (3 passed per target framework)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_CSharp" (net8.0: 312 passed; net9.0: 307 passed, 5 skipped)
• Full solution suite before the final bounded adversarial hardening (net8.0: 12,004 passed, 9 skipped; net9.0: 11,520 passed, 430 skipped)
• dotnet format CodeIndex.sln --no-restore --verify-no-changes --severity warn
• dotnet run --project tools/CodeIndex.Changelog -- check (11 fragments validated)
• git diff --check
• Two bounded codex review --base origin/main rounds completed; all five actionable P2 findings were fixed and their reproductions were added to the regression fixtures.
• Post-commit repository index refresh and root/workspace readiness checks passed with no warnings or errors.

Documentation and changelog

• Updated the English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5192.fixed.md.

Follow-up candidates

None.

Fixes #5192