fix(qq): make start() long-running per base channel contract

QQ channel's start() created a background task and returned immediately,
violating the base Channel contract which specifies start() should be
"a long-running async task". This caused the gateway to exit prematurely
when QQ was the only enabled channel.

Now directly awaits _run_bot() to stay alive like other channels.

Fixes #894

nanobot/channels/qq.py

@@ -71,8 +71,8 @@ class QQChannel(BaseChannel):
         BotClass = _make_bot_class(self)
         self._client = BotClass()
 
-        self._bot_task = asyncio.create_task(self._run_bot())
         logger.info("QQ bot started (C2C private message)")
+        await self._run_bot()
 
     async def _run_bot(self) -> None:
         """Run the bot connection with auto-reconnect."""