Stabilize GitHelper timeout regression test under full-suite load

[Widthdom]

Summary

• keep the fake diff-tree process blocked beyond the assertion window and record its PID
• verify the 500 ms timeout reaps the fake Git process before returning within the shared 10-second wall-clock bound
• document the full-suite timing contract and add a bilingual changelog fragment

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet build CodeIndex.sln -c Debug --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• focused timeout test passed repeatedly, including under concurrent index/full-suite load
• GitHelperTests: 77 passed on Release net8.0
• full Release suite with 5-minute hang collection:
  • net9.0: 10,612 passed, 420 skipped, 0 failed
  • net8.0: the changed timeout test passed; 11,085 passed and 7 skipped, with one unrelated RunBatch_ParallelRefreshesDirectSessionsBetweenItems_Issue4872 load-time failure; its immediate isolated rerun passed
• dotnet tools/CodeIndex.Changelog/bin/Release/net8.0/CodeIndex.Changelog.dll check (101 fragments validated)
• cdidx status --check --json (index_matches_workspace: true, reference graph complete)
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• updated the English and Japanese timeout-test guidance in TESTING_GUIDE.md
• added changelog.d/unreleased/4982.internal.md

Follow-up candidates

• None.

Fixes #4982