Fix #571: drain dead worker queues before closing them

[nitrobass24]

Summary

Fixes #571. WorkerSupervisor.recreate_if_dead() swapped in a fresh worker and then closed the dead worker's result queues without draining them first, so extract/validate work the worker finished just before crashing was silently dropped — the sole consumer (ModelUpdater) read the fresh (empty) worker's queues after the swap.

The fix

• Carry-over buffer in the supervisor. _drain_dead(dead) drains the dead worker's pop_completed/pop_failed queues into a buffer before _reap_dead() closes them (best-effort, per-queue isolated, inside try/finally so FDs are always freed).
• Explicit pop_completed/pop_failed on the supervisor return buffered + fresh, then clear — surfacing carried results exactly once, ahead of the fresh worker's. Buffers extend so rapid back-to-back restarts accumulate rather than clobber.
• ModelUpdater reads completed/failed through the supervisor (drops .worker); statuses stay on .worker (display-only).

Adversarial-review hardening

• Read the live worker before clearing the buffer, so a failed live read can't drop the carry-over (it survives to a later cycle).
• try/finally guarantees the dead worker's queues are closed even if draining raises.

Honest limitation

Items stuck in a dead child's unflushed multiprocessing.Queue feeder thread are unrecoverable; we recover everything the parent can still read — consistent with the #535 "redo, don't guarantee" recovery contract.

Note for reviewers

ModelUpdater previously read completed/failed via supervisor.worker.pop_*(); it now goes through the supervisor so buffered results are surfaced. The one existing test that asserted the dead worker is never read (test_pipeline_recreate_rewires_updater_reads) was updated to reflect the intended new behavior (dead worker drained exactly once).

Test plan

• test_recreate_drains_dead_worker_results_before_close — fails before fix (returns []), passes after.
• Added: exactly-once surfacing, rapid-restart accumulation, per-queue drain-failure isolation, buffer survival on a failing live read, reap-runs-on-drain-failure.
• 494 controller unit tests pass (489 baseline + 5 new), 0 regressions.
• pyright strict: 0 errors on worker_supervisor.py + model_updater.py.
• ruff + C901 clean; methods within code-health targets.

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📥 Commits

Reviewing files that changed from the base of the PR and between 55be236 and 4198c81.

📒 Files selected for processing (3)

• src/python/controller/model_updater.py
• src/python/controller/worker_supervisor.py
• src/python/tests/unittests/test_controller/test_worker_supervisor.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/571-drain-dead-worker-queues

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