fix: capture exception in federation bell clear warning log

Captures the exception as 'exc' in the except clause and includes it
as the third argument to the warning log message. This ensures that
the failure reason is observable in logs, improving diagnostics for
federation heartbeat failures.

Generated with Amplifier

Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

muxplex/main.py

@@ -165,11 +165,12 @@ async def _run_poll_cycle() -> None:
                                     bell_clear_url,
                                     headers={"Authorization": f"Bearer {remote_key}"} if remote_key else {},
                                 )
-                            except Exception:
+                            except Exception as exc:
                                 _log.warning(
-                                    "federation bell clear failed for %s: %s",
+                                    "federation bell clear failed for %s at %s: %s",
                                     viewing_session,
                                     bell_clear_url,
+                                    exc,
                                 )
 
         # 10. Prune devices that haven't sent a heartbeat recently