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.