Fix GitHelper cancellation test threshold

[Widthdom]

Summary

• Relax GitHelper cancellation wall-clock assertions from 2 seconds to a shared 4-second bound that still stays below the fake git scripts' 5-second natural completion.
• Document the GitHelper cancellation test contract in both English and Japanese sections of TESTING_GUIDE.md.
• Add changelog fragment changelog.d/unreleased/3643.fixed.md.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~GitHelperTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Updated TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/3643.fixed.md.
• Did not edit CHANGELOG.md.

Follow-up candidates

• None.

Fixes #3643