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/platforms/discord.py

@@ -66,7 +66,7 @@ if DISCORD_AVAILABLE and _discord_module is not None:
 
         async def on_ready(self) -> None:
             """Called when the bot is ready."""
-            self._platform._set_connected(True)
+            self._platform._connected = True
             logger.info("Discord platform connected")
 
         async def on_message(self, message: Any) -> None:
@@ -118,10 +118,6 @@ class DiscordPlatform(MessagingPlatform):
         self._pending_voice: dict[tuple[str, str], tuple[str, str]] = {}
         self._pending_voice_lock = asyncio.Lock()
 
-    def _set_connected(self, connected: bool) -> None:
-        """Update connection state via an explicit accessor."""
-        self._connected = connected
-
     async def _handle_client_message(self, message: Any) -> None:
         """Adapter entry point used by the internal discord client."""
         await self._on_discord_message(message)
@@ -367,7 +363,7 @@ class DiscordPlatform(MessagingPlatform):
     async def stop(self) -> None:
         """Stop the bot."""
         if self._client.is_closed():
-            self._set_connected(False)
+            self._connected = False
             return
 
         await self._client.close()
@@ -379,7 +375,7 @@ class DiscordPlatform(MessagingPlatform):
                 with contextlib.suppress(asyncio.CancelledError):
                     await self._start_task
 
-        self._set_connected(False)
+        self._connected = False
         logger.info("Discord platform stopped")
 
     async def send_message(