fix(gateway): add metadata param to _keep_typing and base send_typing

_keep_typing() was called with metadata= for thread-aware typing
indicators, but neither it nor the base send_typing() accepted
that parameter. Most adapter overrides (Slack, Discord, Telegram,
WhatsApp, HA) already accept metadata=None, but the base class
and Signal adapter did not.

- Add metadata=None to BasePlatformAdapter.send_typing()
- Add metadata=None to BasePlatformAdapter._keep_typing(), pass through
- Add metadata=None to SignalAdapter.send_typing()

Fixes TypeError in _process_message_background for Signal.

gateway/platforms/signal.py

@@ -580,7 +580,7 @@ class SignalAdapter(BasePlatformAdapter):
             return SendResult(success=True)
         return SendResult(success=False, error="RPC send failed")
 
-    async def send_typing(self, chat_id: str) -> None:
+    async def send_typing(self, chat_id: str, metadata=None) -> None:
         """Send a typing indicator."""
         params: Dict[str, Any] = {
             "account": self.account,