Stabilize import cancellation console capture

[Widthdom]

Summary

• Serialize the pre-cancelled JSON import fixture with the existing non-parallel console-sensitive collection.
• Keep the pure bounded-stream cancellation check parallel-safe in a separate fixture.
• Add a cross-framework reflection contract that pins the console-sensitive assignment.

Root cause

The test captured process-wide Console.Error while it still ran in parallel with tests that emitted request-id diagnostics without taking the capture lock. Those unrelated diagnostics could therefore enter the captured stderr stream in full-suite runs.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• Focused cancellation and collection-contract tests on net8.0 and net9.0
• Full net8.0 suite: 9,744 passed, 6 skipped, 0 failed
• Full net9.0 suite: 9,279 passed, 410 skipped, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• Updated the English and Japanese shared-state guidance in TESTING_GUIDE.md.
• Added changelog.d/unreleased/4625.fixed.md.
• Added changelog.d/unreleased/4650.fixed.md.

Follow-up candidates

None.

Fixes #4625
Fixes #4650