fix(discord): typing indicator task not cleaned up after API error

When the Discord typing API call fails (rate limit, network error, 403),
_typing_loop returns early but the stale task remains in _typing_tasks.
Subsequent send_typing calls see the stale entry and skip, leaving no
typing indicator for the rest of the agent invocation.

Add finally block to _typing_loop to always remove the task from
_typing_tasks on exit, whether from cancellation, error, or normal
completion. This allows send_typing to create a fresh task.

3 new tests in test_discord_send.py:
- Task removed after API error
- Typing restartable after failure
- stop_typing cleans up

gateway/platforms/discord.py

@@ -2697,6 +2697,8 @@ class DiscordAdapter(BasePlatformAdapter):
                     await asyncio.sleep(8)
             except asyncio.CancelledError:
                 pass
+            finally:
+                self._typing_tasks.pop(chat_id, None)
 
         self._typing_tasks[chat_id] = asyncio.create_task(_typing_loop())