Split oversized test files by feature area

[Widthdom]

Summary

• Split the oversized query command, reference extractor, and symbol extractor test classes into feature-focused partial class files.
• Kept the original shared helpers in the root test files and updated TESTING_GUIDE.md for the new layout.
• Added a changelog fragment for the internal test-layout change.
• Sanitized child CLI subprocess environments in query command tests so ambient cdidx defaults from parallel tests do not leak into subprocess assertions.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~QueryCommandRunnerTests|FullyQualifiedName~ReferenceExtractorTests|FullyQualifiedName~SymbolExtractorTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --environment CDIDX_DEFAULT_LIMIT=6 --filter FullyQualifiedName~RunReferences_ExactJson_SqlNonAsciiBareIdentifiersStayWhole
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review completed; no branch-specific actionable findings after the subprocess environment fix.

Notes

• Related follow-up issue filed for an unrelated intermittent concurrency-suite failure observed during validation: Investigate intermittent ConcurrencyTests SQL logic error #2932.
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Fixes #2817