Fix SQL quoted qualified name matching

[Widthdom]

Summary

• Preserve case-sensitive matching for PostgreSQL double-quoted SQL qualified names while keeping MySQL backtick and T-SQL bracket matching case-insensitive.
• Use the same dialect-aware resolver path for SQL unused-symbol exact matching.
• Add regression coverage and a bilingual changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameReferenceExtractorTests.Extract_Sql|FullyQualifiedNameReferenceExtractorTests.Extract_SQL|FullyQualifiedNameReferenceExtractorTests.SqlNameResolver|FullyQualifiedNameSymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameDbReaderTests.SqlQualifiedNames|FullyQualifiedNameReferenceExtractorTests.SqlNameResolver_QuotedQualifiedNames"
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Added changelog fragment: changelog.d/unreleased/2101.fixed.md
• No README/guide changes required; this is a bug fix to existing SQL matching behavior.

Follow-up candidates

• None.

Fixes #2101