Isolate search-alternative tests from the live repository index

[Widthdom]

Search-alternative tests could read the developer's live repository index before rejecting unsupported search flags. Concurrent index updates reproduced failures in both the language-alias JSON case and the compact-output case on net8.0 and net9.0.

Use a shared temporary database/project context for every invocation in the existing fixture, run it in the SQLite pool sensitive collection, and restore configuration, data-directory environment, and current-directory state in finally. Preserve the existing alternative-command assertions and explicitly assert that JSON replay arguments do not invent --db or --data-dir flags. Production snapshot safety behavior is unchanged.

Validation:

• Build: passed with no warnings or errors.
• Both frameworks: all 10 focused tests passed while the repository index was actively writing; both affected tests failed before the fix under concurrent indexing.
• Repository formatting and changelog validation: passed.
• Full net8.0 suite: 12,257 passed, 9 skipped, 5 unrelated failures during concurrent repository indexing. All five failed cases passed in a focused retry after the writer finished. The applicable envelope case also passed on net9.0 (production-runtime cases are intentionally skipped there).
• Full net9.0 suite: 11,773 passed, 435 skipped, 0 failures.
• Post-commit rebuild and root/workspace index checks: passed; the index matches the committed workspace and reports complete coverage.
• Codex adversarial review: No blocking/actionable issues found.

Updated both language sections in TESTING_GUIDE.md and added changelog.d/unreleased/5266.internal.md. No public CLI behavior changes require README or developer-guide changes.

Follow-up references: #5264, #5270.

Fixes #5266