test(extract): make the fan-out drain test deterministic

[jasonssdev]

Summary

Makes test_concurrent_fan_out_drains_in_flight_windows_before_it_raises deterministic. The old test ordered a slow window against a failing one with time.sleep(0.3), so on a loaded runner window 1 could be cancelled before it started and the drained flag was never set. The new test orders everything with threading.Events: window 0 fails only after window 1 is provably in its call, window 1 stays there until the test releases it, and the test asserts the fan-out is still running while window 1 is in flight, then that it raised OllamaUnavailable after the drain. Test-only; no production change.

Related issue

Closes #1164

Type of change

• Tests

How was this tested?

• New test passes 200 consecutive runs in a shell loop, no failure.
• Mutation: changing pool.shutdown(wait=True, ...) to wait=False in extraction/concept.py turns the test red (assert runner.is_alive(), "fan-out returned with a window in flight"); reverted byte-exactly and __pycache__ purged.
• ruff check ., ruff format --check ., mypy . clean; tests/unit/extraction/test_concept.py 368 passed.

Checklist

• Conventional Commit title with a project scope
• Tests added or updated
• No CHANGELOG entry (test-only)

Notes for reviewers

The bounded runner.join(timeout=0.5) only bounds the failing (mutant) case; with a correct drain the runner cannot finish while window 1 is held, so the assertion cannot flake.

Other time.sleep( used for thread ordering in tests/unit/extraction and tests/unit/cli/test_ingest.py: none. The only remaining match is a docstring at test_concept.py (_WindowKeyedLLM-adjacent test) that explains why a sleep is not used.