Stabilize bounded regex consumer-break test

[Widthdom]

Summary

• give the static custom-timeout consumer-break regression scheduler headroom while keeping the timeout below the production default
• assert the returned prefix and verify that early disposal records no suffix timeout
• document the contract in both TESTING_GUIDE languages and add the Issue BoundedRegex consumer-break test can flake during the full parallel suite #5113 changelog fragment

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~BoundedRegexTests.EnumerateMatches_StaticPatternCustomTimeout_StopsAfterConsumerBreak (net8.0 and net9.0: 1 passed each)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-restore --filter FullyQualifiedName~CodeIndex.Tests.BoundedRegexTests (net8.0 and net9.0: 23 passed each)
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore --no-build --logger "console;verbosity=minimal" (net8.0: 11,738 passed, 7 skipped; net9.0: 11,259 passed, 423 skipped; 0 failed)
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false -m:1
• root and workspace index checks both healthy after the post-commit --commits refresh
• Codex adversarial review: no blocking/actionable diff findings

Documentation and changelog

• updated the English and Japanese sections of TESTING_GUIDE.md
• added changelog.d/unreleased/5113.fixed.md

Follow-up candidates

• Git timeout regression test cannot reach diff-tree after single-commit validation #5130
• LocalJsonl source guard can cross parallel unreadable test markers #5142

Fixes #5113