fix: store asyncio task references to prevent GC mid-execution (#3267)

Python's asyncio event loop holds only weak references to tasks.
Without a strong reference, the garbage collector can destroy a task
while it's awaiting I/O — silently dropping messages. Python 3.12+
made this more aggressive.

Audit of all gateway platform adapters found 6 untracked create_task
calls across 6 files:

Per-message tasks (tracked via _background_tasks set from base class):
- gateway/platforms/webhook.py: handle_message task
- gateway/platforms/sms.py: handle_message task
- gateway/platforms/signal.py: SSE response aclose task

Long-running infrastructure tasks (stored in named instance vars):
- gateway/platforms/slack.py: Socket Mode handler (_socket_mode_task)
- gateway/platforms/discord.py: bot client (_bot_task)
- gateway/platforms/whatsapp.py: message poll loop (_poll_task, 2 sites)

All other adapters (telegram, mattermost, matrix, email, homeassistant,
dingtalk) already tracked their tasks correctly.

Salvaged from PR #3160 by memosr — expanded from 1 file to 6.

gateway/platforms/slack.py

@@ -72,6 +72,7 @@ class SlackAdapter(BasePlatformAdapter):
         self._handler: Optional[AsyncSocketModeHandler] = None
         self._bot_user_id: Optional[str] = None
         self._user_name_cache: Dict[str, str] = {}  # user_id → display name
+        self._socket_mode_task: Optional[asyncio.Task] = None
 
     async def connect(self) -> bool:
         """Connect to Slack via Socket Mode."""
@@ -119,7 +120,7 @@ class SlackAdapter(BasePlatformAdapter):
 
             # Start Socket Mode handler in background
             self._handler = AsyncSocketModeHandler(self._app, app_token)
-            asyncio.create_task(self._handler.start_async())
+            self._socket_mode_task = asyncio.create_task(self._handler.start_async())
 
             self._running = True
             logger.info("[Slack] Connected as @%s (Socket Mode)", bot_name)