Prune generated outputs from LocalJsonl source guard

[Widthdom]

Summary

• prune bin and obj directories before the LocalJsonl repository source guard enumerates their descendants
• add deterministic traversal-seam coverage that preserves exhaustive enumeration of intended C# source files
• document the parallel-test contract in both English and Japanese

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~LocalJsonlJsonWriterOptionsTests --logger "console;verbosity=minimal"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-restore -m:1 -p:UseSharedCompilation=false --filter FullyQualifiedName~LocalJsonlJsonWriterOptionsTests --logger "console;verbosity=minimal"
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore --logger "console;verbosity=minimal"
  • net8.0: 11,760 passed, 7 skipped, 0 failed
  • net9.0: 11,281 passed, 423 skipped, 0 failed
• dotnet build CodeIndex.sln -c Release --no-restore -m:1 -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• updated the English and Japanese parallelism guidance in TESTING_GUIDE.md
• added changelog.d/unreleased/5142.fixed.md

Follow-up candidates

• None.

Fixes #5142