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/whatsapp.py

@@ -140,6 +140,7 @@ class WhatsAppAdapter(BasePlatformAdapter):
         self._message_queue: asyncio.Queue = asyncio.Queue()
         self._bridge_log_fh = None
         self._bridge_log: Optional[Path] = None
+        self._poll_task: Optional[asyncio.Task] = None
     
     async def connect(self) -> bool:
         """
@@ -198,7 +199,7 @@ class WhatsAppAdapter(BasePlatformAdapter):
                                 print(f"[{self.name}] Using existing bridge (status: {bridge_status})")
                                 self._mark_connected()
                                 self._bridge_process = None  # Not managed by us
-                                asyncio.create_task(self._poll_messages())
+                                self._poll_task = asyncio.create_task(self._poll_messages())
                                 return True
                             else:
                                 print(f"[{self.name}] Bridge found but not connected (status: {bridge_status}), restarting")
@@ -304,7 +305,7 @@ class WhatsAppAdapter(BasePlatformAdapter):
                     print(f"[{self.name}]   If session expired, re-pair: hermes whatsapp")
             
             # Start message polling task
-            asyncio.create_task(self._poll_messages())
+            self._poll_task = asyncio.create_task(self._poll_messages())
             
             self._mark_connected()
             print(f"[{self.name}] Bridge started on port {self._bridge_port}")