Isolate global log report-hint test from ambient WAL changes

[Widthdom]

The global-log timestamp/stack-trace test injected its failure before query dispatch but still resolved the ambient database while persisting last-failure provenance. A changing WAL can therefore suppress the report hint even when lifecycle logging succeeds.

The test now pins a private, absent --db, scopes configuration discovery and text-log provenance, and restores its environment, culture, and snapshot hook. One shared theory covers stable and continuously changing ambient WALs, verifies the three-attempt query_only_wal_changed boundary, and retains the report-hint, UTC, and stack-trace assertions. Production behavior is unchanged. This establishes a deterministic interference mechanism, not proof of the historical run's cause or process-state leakage.

Validation:

• Before the DB pin, the changing-WAL case reproduced the exact missing-report-hint assertion on both net8.0 and net9.0; the stable control passed on both.
• After the fix, all 36 GlobalToolLogTests cases passed on each framework.
• Full suites, with no concurrent dogfood index writes: net8.0 passed 12,263 / skipped 9; net9.0 passed 11,774 / skipped 435; zero failures on either framework.
• After integrating the latest main and preserving both testing-guide entries at the conflict, the combined logging and search-alternative suites passed all 46 cases on each framework. The full runs above preceded this upstream test-only change.
• Solution build, dotnet format CodeIndex.sln --verify-no-changes --no-restore, and bilingual fragment validation passed.
• Codex adversarial review of origin/main..HEAD: No blocking/actionable issues found.

Documentation: updated the English and Japanese sections of TESTING_GUIDE.md; added changelog.d/unreleased/5264.internal.md. No production documentation or agent instruction changes are needed.

Fixes #5264