Remove over-engineering: drop tree_queue setter, _set_connected(), fi… (#63)

…x cancel_all() TOCTOU

- Remove tree_queue property setter (backward-compat hack; all callers
already migrated to replace_tree_queue()); keep property getter only
- Update 2 remaining tests that still used direct assignment to use
replace_tree_queue()
- Remove _set_connected() 1-line wrapper on DiscordPlatform; assign
_connected directly
- Fix cancel_all() TOCTOU: hold self._lock for the full loop so newly
created trees cannot slip through between the snapshot and cancellation

---------

Co-authored-by: Claude <noreply@anthropic.com>

messaging/trees/queue_manager.py

@@ -314,11 +314,10 @@ class TreeQueueManager:
         """Cancel all messages in all trees."""
         async with self._lock:
             root_ids = list(self._repository.tree_ids())
-
-        all_cancelled: list[MessageNode] = []
-        for root_id in root_ids:
-            all_cancelled.extend(await self.cancel_tree(root_id))
-        return all_cancelled
+            all_cancelled: list[MessageNode] = []
+            for root_id in root_ids:
+                all_cancelled.extend(await self.cancel_tree(root_id))
+            return all_cancelled
 
     def cleanup_stale_nodes(self) -> int:
         """