Stabilize MCP timeout lease test under full-suite load

[Widthdom]

Summary

• Give the timeout-lease regression scheduler headroom and ID-specific readiness signals.
• Replace instantaneous and null-forgiving checks with bounded blocked-state and typed JSON assertions.
• Document the bilingual test contract and add the changelog fragment.

Root cause

Under full-suite load, the 100 ms timeout could expire while the second ping response continuation was scheduler-delayed. The test then followed a missing result node with null-forgiving operators, masking the timing failure as a NullReferenceException.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• dotnet test CodeIndex.sln -c Release --no-build -p:UseSharedCompilation=false
  • net8.0: 10,376 passed, 7 skipped, 0 failed
  • net9.0: 9,918 passed, 404 skipped, 0 failed
• Target test: five consecutive passes on both net8.0 and net9.0, plus a final pass on both frameworks after updating to the latest origin/main
• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no actionable regressions

Documentation and changelog

• Updated the English and Japanese sections of TESTING_GUIDE.md.
• Added changelog.d/unreleased/4807.fixed.md.

Follow-up candidates

None.

Fixes #4807