fix: clear stale errors on successful poll — prevents phantom error alerts

Same fix as applied to the pi Zulip extension: last_error_msg and
last_error_at are now cleared on every successful poll cycle, not
just on reconnect. Health monitors no longer show stale errors.

plugins/platforms/zulip/adapter.py

@@ -449,6 +449,10 @@ class ZulipAdapter(BasePlatformAdapter):
                             except Exception:
                                 pass
 
+                # Clear stale errors on successful poll cycle
+                self._health_stats["last_error_msg"] = None
+                self._health_stats["last_error_at"] = None
+
                 # Gen 4: Heartbeat logging — prove poll loop is alive
                 now = time.time()
                 if now - self._last_heartbeat_time > HEARTBEAT_INTERVAL: