Follow-up to the cherry-picked watchdog from #112052 (@KoNit-K), finishing the
class the reporter of #112049 laid out:
- `_websocket_loop` runs the read loop and the discovery sweep as sibling
tasks and ends the connection when EITHER finishes. The discovery sweep
re-raises `ConnectionClosed` instead of logging it and retrying next tick:
a send that sees the socket closed is proof the read the loop is parked on
will never return. That is exactly the traceback the reporter watched for
22-86 h while inbound stayed silent.
- Health is invalidated while reconnecting: the first disconnect publishes
`retrying` (`_mark_degraded`) and a successful re-subscribe publishes
`connected` again. Before, `connect()` wrote "connected" once and nothing
ever changed it, so `/health/detailed` claimed delivery during the silence.
- The teardown awaits both tasks with `gather(return_exceptions=True)` instead
of a bare `except (CancelledError, Exception): pass`, which could swallow a
`disconnect()` cancellation landing mid-teardown.
- Slims the salvaged read-loop hunk: the extra "receive task remained parked"
warning and the in-loop `_mark_degraded()` are dropped; the reconnect log
line and the loop-level health flip cover both.
Docs: the Buzz page still described inbound as poll-only and the WebSocket
transport as a future optimization; it now describes the watchdog and the
`retrying` health state.
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