Stabilize parallel batch snapshot refresh tests

[Widthdom]

Summary

• run the interactive parallel-batch refresh fixtures on a dedicated test thread and await first-wave completion asynchronously
• replace broad status queries with lightweight JSON file-count queries while preserving detached/direct generation-refresh assertions
• document the deterministic fixture contract and add the bilingual changelog fragment

Root cause

The fixture blocked a thread-pool thread while its outer batch loop, input pump, producer, and workers also depended on the pool. It additionally used four broad status --json queries when only the indexed file count was needed, making the net8 test vulnerable to suite load and masking the original boundary during cleanup.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net8.0 --no-build --no-restore --filter "FullyQualifiedName~Issue4872" (6 passed)
• the same issue-focused test group on net9.0 (6 passed)
• full net8.0 test project (11,096 passed, 7 skipped, 0 failed)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore --include tests/CodeIndex.Tests/QueryCommandRunnerBatchIssue4872Tests.cs
• dotnet run --project tools/CodeIndex.Changelog -- check
• cdidx status --check --json (index_matches_workspace: true)
• mandatory Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• updated the English and Japanese sections of TESTING_GUIDE.md
• added changelog.d/unreleased/5033.internal.md

Follow-up candidates

• None.

Fixes #5033