test(kanban): cover send-exception rewind + drop noisy success log to debug

Two follow-up improvements to the previous commit's notifier dedup work.

1. Add a regression test for the send-exception rewind path. The
   contributor's PR included a test for the adapter-disconnect path
   (test_kanban_notifier_rewinds_claim_if_adapter_disconnects, where
   adapter is None at delivery time), but not for the "adapter is
   connected, send() raises" path that fires inside the inner try/except
   at gateway/run.py:4314. The new test
   (test_kanban_notifier_rewinds_claim_on_send_exception) uses a
   FailingAdapter that always raises and confirms (a) send was actually
   attempted, (b) the claim was rewound, (c) the next call to
   unseen_events_for_sub still returns the event for retry.

2. Drop the per-delivery success log from INFO to DEBUG. A busy board
   on a multi-platform gateway can produce hundreds of these per day;
   that's gateway.log noise that obscures real warnings. Failure paths
   stay at WARNING (where you'd want to look when something's wrong)
   so we don't lose visibility into transient send issues.

gateway/run.py

@@ -4303,7 +4303,7 @@ class GatewayRunner:
                             await adapter.send(
                                 sub["chat_id"], msg, metadata=metadata,
                             )
-                            logger.info(
+                            logger.debug(
                                 "kanban notifier: delivered %s event for %s to %s/%s on board %s",
                                 kind, sub["task_id"], platform_str, sub["chat_id"], board_slug,
                             )