Fix C# function extraction after literal delimiters

[Widthdom]

Summary

• reuse the full C# lexer's raw-column-aligned masking for declaration, callable-parameter, type-body, and switch-expression scope scans
• prevent delimiters inside character/string literals and comments from suppressing later C# methods, while preserving declaration ranges and reference containers
• bump the C# extractor contract so existing indexed C# files are automatically re-extracted
• add direct extractor, real-index query, long-tail, reference false-positive, and bounded-regex-timeout regressions

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build --no-restore (net8.0: 11,921 passed, 9 skipped; net9.0: 11,439 passed, 428 skipped; 0 failed)
• focused Issue Prevent C# function extraction from silently stopping partway through a file #5182 tests on net8.0 and net9.0 (4 passed per target, 0 failed)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• repository dogfood index/status/workspace checks, including exact symbol verification for the methods reported in the issue
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• extend the bilingual C# declaration-boundary test contract in TESTING_GUIDE.md
• add bilingual fragment changelog.d/unreleased/5182.fixed.md

Follow-up candidates

None.

Fixes #5182