Stabilize console restoration assertions

[Widthdom]

Summary

• Serialize the timeout test's global console writer snapshot and restoration assertions with the shared TestConsoleLock.Gate ownership gate.
• Document the convention in the English and Japanese test guidance.
• Add changelog.d/unreleased/4800.fixed.md.

Root cause

ConsoleCapture.CaptureAsync restored the original writers and released console ownership before the test read Console.Out and Console.Error. A parallel console-mutating test could therefore install a different writer between restoration and the assertions, causing a full-suite-only race.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• ConsoleCaptureTests on net8.0 and net9.0: 8/8 passed on each target framework
• The affected timeout test repeated successfully on both target frameworks
• Full suite: net8.0 passed (10,242 passed, 7 skipped); net9.0 completed with one unrelated transient failure in CdidxConfigFileTests.RunValidate_PositionalJson_ReturnsStructuredError_Issue3892 (9,783 passed, 404 skipped), and that exact test passed when rerun on both target frameworks
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• Updated both language sections of TESTING_GUIDE.md.
• Added changelog fragment changelog.d/unreleased/4800.fixed.md.

Follow-up candidates

• None.

Fixes #4800