Stabilize Docker entrypoint contract timeout

[Widthdom]

Summary

• Increase the Docker entrypoint contract fixture's bounded process deadline from 5 seconds to 30 seconds so normal concurrent full-suite contention does not cause a false timeout.
• Preserve process-tree termination when the fixture genuinely exceeds the deadline.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore (net8.0: 11,749 passed, 7 skipped; net9.0: 11,270 passed, 423 skipped; 0 failed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net8.0 --no-restore --filter "FullyQualifiedName~ReleaseWorkflow_DockerEntrypoint" --logger "console;verbosity=normal"
• dotnet build CodeIndex.sln -c Release --no-restore
• make lint
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root and workspace cdidx status --check health checks
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Updated both the English and Japanese Docker entrypoint test guidance in TESTING_GUIDE.md.
• Added bilingual fragment changelog.d/unreleased/5132.fixed.md.

Follow-up candidates

• None.

Fixes #5132