Fix CSS media feature symbol extraction

[Widthdom]

Summary

• index CSS @media feature names as property symbols
• skip media feature literal values and boolean operators
• add CSS symbol extraction coverage
• fix Rust associated type default extraction for trait containers stored as protocol symbols

Documentation / changelog

• Added changelog fragment: changelog.d/unreleased/2105.fixed.md
• Added changelog fragment: changelog.d/unreleased/2611.fixed.md

Fixes #2105
Fixes #2611

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_CSS_CapturesMediaFeatureNamesButNotValuesOrOperators"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net9.0 --filter "FullyQualifiedName~SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols"
• codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

• None.

Notes

• The Rust fix addresses the Build and Test net9.0 CI failure observed on this PR after issue Rust associated type defaults are skipped under protocol-kind traits #2611 was filed.