Fix issue #363: add exact regression coverage for C# phantom string symbols

[Widthdom]

Summary

Fixes #363.

This PR adds exact regression coverage for the user-reported C# phantom-symbol fixture involving raw strings, interpolated raw strings, and multi-line verbatim strings.

What changed

• Added a SymbolExtractorTests exact repro for issue C# raw string phantom symbols persist: """...""", $"""...""", and @"..." content still captured as real symbols (#177 regression) #363.
• Added a QueryCommandRunnerTests end-to-end RunSymbols regression for the same fixture.
• Verified that the current local binary already returns the expected 7 symbols for the issue fixture and locked that behavior in tests.

Why

The underlying masking behavior is already present on current main, but issue #363's exact dogfood fixture was not pinned directly in the test suite. This PR prevents silent regression of that specific report.

Validation

• dotnet test
• Local repro with dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index ... and symbols --db ... --json
• Adversarial Codex review on origin/main...HEAD with no blocking/actionable findings