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/webhook.py

@@ -363,7 +363,9 @@ class WebhookAdapter(BasePlatformAdapter):
         )
 
         # Non-blocking — return 202 Accepted immediately
-        asyncio.create_task(self.handle_message(event))
+        task = asyncio.create_task(self.handle_message(event))
+        self._background_tasks.add(task)
+        task.add_done_callback(self._background_tasks.discard)
 
         return web.json_response(
             {