feat(discord): persistent typing indicator for DMs

Based on PR #2427 by @oxngon (core feature extracted, reformatting
and unrelated changes dropped).

Discord's TYPING_START gateway event is unreliable for bot DMs. This
adds a background typing loop that hits POST /channels/{id}/typing
every 8 seconds (indicator lasts ~10s) until the response is sent.

- send_typing() starts a per-channel background loop (idempotent)
- stop_typing() cancels it (called after _run_agent returns)
- Base adapter gets stop_typing() as a no-op default
- Per-channel tracking via _typing_tasks dict prevents duplicates

gateway/platforms/base.py

@@ -504,6 +504,14 @@ class BasePlatformAdapter(ABC):
         metadata: optional dict with platform-specific context (e.g. thread_id for Slack).
         """
         pass
+
+    async def stop_typing(self, chat_id: str) -> None:
+        """Stop a persistent typing indicator (if the platform uses one).
+
+        Override in subclasses that start background typing loops.
+        Default is a no-op for platforms with one-shot typing indicators.
+        """
+        pass
     
     async def send_image(
         self,