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/discord.py

@@ -446,6 +446,7 @@ class DiscordAdapter(BasePlatformAdapter):
         # Persistent typing indicator loops per channel (DMs don't reliably
         # show the standard typing gateway event for bots)
         self._typing_tasks: Dict[str, asyncio.Task] = {}
+        self._bot_task: Optional[asyncio.Task] = None
         # Cap to prevent unbounded growth (Discord threads get archived).
         self._MAX_TRACKED_THREADS = 500
     
@@ -588,7 +589,7 @@ class DiscordAdapter(BasePlatformAdapter):
             self._register_slash_commands()
             
             # Start the bot in background
-            asyncio.create_task(self._client.start(self.config.token))
+            self._bot_task = asyncio.create_task(self._client.start(self.config.token))
             
             # Wait for ready
             await asyncio.wait_for(self._ready_event.wait(), timeout=30)