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/run.py

@@ -2125,7 +2125,15 @@ class GatewayRunner:
                 session_id=session_entry.session_id,
                 session_key=session_key
             )
-            
+
+            # Stop persistent typing indicator now that the agent is done
+            try:
+                _typing_adapter = self.adapters.get(source.platform)
+                if _typing_adapter and hasattr(_typing_adapter, "stop_typing"):
+                    await _typing_adapter.stop_typing(source.chat_id)
+            except Exception:
+                pass
+
             response = agent_result.get("final_response") or ""
             agent_messages = agent_result.get("messages", [])
 
@@ -2325,6 +2333,13 @@ class GatewayRunner:
             return response
             
         except Exception as e:
+            # Stop typing indicator on error too
+            try:
+                _err_adapter = self.adapters.get(source.platform)
+                if _err_adapter and hasattr(_err_adapter, "stop_typing"):
+                    await _err_adapter.stop_typing(source.chat_id)
+            except Exception:
+                pass
             logger.exception("Agent error in session %s", session_key)
             error_type = type(e).__name__
             error_detail = str(e)[:300] if str(e) else "no details available"