fix(wecom): guard flush task against cancel-delivery race to prevent message loss

When asyncio.sleep() fires just before Task.cancel() is called, CPython
sets _must_cancel=True but cannot cancel the already-completed sleep
future, so CancelledError is delivered at the next await (handle_message)
rather than at the sleep.  By that point the superseded task has already
popped the merged event from _pending_text_batches, so the superseding
task sees an empty batch and silently drops the message.

Fix: add a synchronous task-registry check between the sleep and the pop.
No await between the check and the pop means no other coroutine can
interleave, so the guard is race-free.

gateway/platforms/wecom.py

@@ -616,6 +616,18 @@ class WeComAdapter(BasePlatformAdapter):
             else:
                 delay = self._text_batch_delay_seconds
             await asyncio.sleep(delay)
+            # Guard against the cancel-delivery race: when the sleep timer
+            # fires just before cancel() is called, CPython sets
+            # Task._must_cancel but cannot cancel the already-done sleep
+            # future, so CancelledError is delivered at the *next* await
+            # (handle_message) rather than here.  By that point this task
+            # has already popped the merged event, so the superseding task
+            # sees an empty batch and silently drops the message.
+            # This check is synchronous — no await between the sleep and
+            # the pop — so no other coroutine can modify the task registry
+            # in between.
+            if self._pending_text_batch_tasks.get(key) is not current_task:
+                return
             event = self._pending_text_batches.pop(key, None)
             if not event:
                 return