Uncovered pre-existing branches in the Redis backend

Found by `testJoe` while adding coverage for the fair multi-queue rotation (`codingjoe-fair-multi-queue-scheduling`), and confirmed pre-existing (untouched by that diff):

`defer: pre-existing uncovered lines threadmill/backends/redis.py L147-148, L445, L494-495. Unrelated to this diff. [threadmill/backends/redis.py]`

The four lines are:

- `L147-148` — the `ValueError` raised when `REDIS_URL` is missing from the backend params.
- `L445` — `raise TaskResultDoesNotExist(...)` in `get_result()`.
- `L494-495` — the `except ValueError: continue` that drops malformed pub/sub payloads in `worker_telemetry()`.

None are reachable from the current test suite, so `uv run pytest` reports them as missing. Add direct tests (a backend without `REDIS_URL`, an unknown result id, a malformed telemetry message) or delete the branches that no caller can reach.

Start in threadmill/backends/redis.py at lines 147-148, 445, and 494-495, then inspect the existing Redis backend tests. Run `uv run pytest` to confirm the uncovered branches, and add direct coverage for missing REDIS_URL, an unknown result id, and malformed telemetry; done means the tests pass without those lines remaining uncovered.

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