Replace `with suppress(Exception)` in `_poll_once` message processing
and the `start()` poll loop with explicit `try/except` blocks that
log errors via `logger.exception`. Previously, any exception during
message processing (e.g. in `_handle_message`) was swallowed silently,
causing inbound messages to disappear without a trace.
Also add tests verifying that:
- `_poll_once` logs and continues when `_process_message` fails
- the poll loop logs and continues when `_poll_once` fails
2a318d6991