fix(discord): shield text-batch flush from follow-up cancel (#12444)

When Discord splits a long message at 2000 chars, _enqueue_text_event
buffers each chunk and schedules a _flush_text_batch task with a
short delay.  If another chunk lands while the prior flush task is
already inside handle_message, _enqueue_text_event calls
prior_task.cancel() — and without asyncio.shield, CancelledError
propagates from the flush task into handle_message → the agent's
streaming request, aborting the response the user was waiting on.

Reproducer: user sends a 3000-char prompt (split by Discord into 2
messages).  Chunk 1 lands, flush delay starts, chunk 2 lands during
the brief window when chunk 1's flush has already committed to
handle_message.  Agent's current streaming response is cancelled
with CancelledError, user sees a truncated or missing reply.

Fix (gateway/platforms/discord.py):
- Wrap the handle_message call in asyncio.shield so the inner
  dispatch is protected from the outer task's cancel.
- Add an except asyncio.CancelledError clause so the outer task
  still exits cleanly when cancel lands during the sleep window
  (before the pop) — semantics for that path are unchanged.

The new flush task spawned by the follow-up chunk still handles its
own batch via the normal pending-message / active-session machinery
in base.py, so follow-ups are not lost.

Tests: tests/gateway/test_text_batching.py —
test_shield_protects_handle_message_from_cancel.  Tracks a distinct
first_handle_cancelled event so the assertion fails cleanly when the
shield is missing (verified by stashing the fix and re-running).

Live E2E on the live-loaded DiscordAdapter:
  first_handle_cancelled: False  (shield worked)
  first_handle_completed: True   (handle_message ran to completion)

gateway/platforms/discord.py

@@ -3265,7 +3265,20 @@ class DiscordAdapter(BasePlatformAdapter):
                 "[Discord] Flushing text batch %s (%d chars)",
                 key, len(event.text or ""),
             )
-            await self.handle_message(event)
+            # Shield the downstream dispatch so that a subsequent chunk
+            # arriving while handle_message is mid-flight cannot cancel
+            # the running agent turn.  _enqueue_text_event always cancels
+            # the prior flush task when a new chunk lands; without this
+            # shield, CancelledError would propagate from our task down
+            # into handle_message → the agent's streaming request,
+            # aborting the response the user was waiting on.  The new
+            # chunk is handled by the fresh flush task regardless.
+            await asyncio.shield(self.handle_message(event))
+        except asyncio.CancelledError:
+            # Only reached if cancel landed before the pop — the shielded
+            # handle_message is unaffected either way.  Let the task exit
+            # cleanly so the finally block cleans up.
+            pass
         finally:
             if self._pending_text_batch_tasks.get(key) is current_task:
                 self._pending_text_batch_tasks.pop(key, None)