Uncovered pre-existing branches in the executor and worker command

Found by `testJoe` while adding coverage for the fair multi-queue rotation (`codingjoe-fair-multi-queue-scheduling`). All pre-existing, all untouched by that diff. Filed as one item because none of them is worth its own tracker entry.

- `executor.py:278-280` — the `Empty`/`TimeoutError` handler in `WorkerThread.run`. Currently shadowed by the `redis.py:297` thread crash (see the sibling issue about `None` poll intervals), so it never runs in the suite.
- `management/commands/threadmill.py:114` — invalid-backend `CommandError`.
- `management/commands/threadmill.py:122-123,125` — invalid-log-format `CommandError`.
- `management/commands/threadmill.py:153-156` — `KeyboardInterrupt` shutdown branch.
- `management/commands/threadmill.py:176-177` — missing-inspector-dependency `CommandError`.

Add direct tests for each, or delete the branches no caller can reach. `uv run pytest` reports them as missing today.

Start by reading executor.py:278-280 and management/commands/threadmill.py at lines 114, 122-123, 125, 153-156, and 176-177. Run `uv run pytest` to reproduce the missing coverage, then add direct tests for each branch or confirm which unreachable branches can be removed. Done means the relevant branches are covered or deleted and the test suite passes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.