fix(gateway): cancel_background_tasks must drain late-arrivals (#12471)

During gateway shutdown, a message arriving while
cancel_background_tasks is mid-await (inside asyncio.gather) spawns
a fresh _process_message_background task via handle_message and adds
it to self._background_tasks.  The original implementation's
_background_tasks.clear() at the end of cancel_background_tasks
dropped the reference; the task ran untracked against a disconnecting
adapter, logged send-failures, and lingered until it completed on
its own.

Fix: wrap the cancel+gather in a bounded loop (MAX_DRAIN_ROUNDS=5).
If new tasks appeared during the gather, cancel them in the next
round.  The .clear() at the end is preserved as a safety net for
any task that appeared after MAX_DRAIN_ROUNDS — but in practice the
drain stabilizes in 1-2 rounds.

Tests: tests/gateway/test_cancel_background_drain.py — 3 cases.
- test_cancel_background_tasks_drains_late_arrivals: spawn M1, start
  cancel, inject M2 during M1's shielded cleanup, verify M2 is
  cancelled.
- test_cancel_background_tasks_handles_no_tasks: no-op path still
  terminates cleanly.
- test_cancel_background_tasks_bounded_rounds: baseline — single
  task cancels in one round, loop terminates.

Regression-guard validated: against the unpatched implementation,
the late-arrival test fails with exactly the expected message
('task leaked').  With the fix it passes.

Blast radius is shutdown-only; the audit classified this as MED.
Shipping because the fix is small and the hygiene is worth it.

While investigating the audit's other MEDs (busy-handler double-ack,
Discord ExecApprovalView double-resolve, UpdatePromptView
double-resolve), I verified all three were false positives — the
check-and-set patterns have no await between them, so they're
atomic on single-threaded asyncio.  No fix needed for those.

gateway/platforms/base.py

@@ -2033,12 +2033,26 @@ class BasePlatformAdapter(ABC):
         Used during gateway shutdown/replacement so active sessions from the old
         process do not keep running after adapters are being torn down.
         """
-        tasks = [task for task in self._background_tasks if not task.done()]
-        for task in tasks:
-            self._expected_cancelled_tasks.add(task)
-            task.cancel()
-        if tasks:
+        # Loop until no new tasks appear.  Without this, a message
+        # arriving during the `await asyncio.gather` below would spawn
+        # a fresh _process_message_background task (added to
+        # self._background_tasks at line ~1668 via handle_message),
+        # and the _background_tasks.clear() at the end of this method
+        # would drop the reference — the task runs untracked against a
+        # disconnecting adapter, logs send-failures, and may linger
+        # until it completes on its own.  Retrying the drain until the
+        # task set stabilizes closes the window.
+        MAX_DRAIN_ROUNDS = 5
+        for _ in range(MAX_DRAIN_ROUNDS):
+            tasks = [task for task in self._background_tasks if not task.done()]
+            if not tasks:
+                break
+            for task in tasks:
+                self._expected_cancelled_tasks.add(task)
+                task.cancel()
             await asyncio.gather(*tasks, return_exceptions=True)
+            # Loop: late-arrival tasks spawned during the gather above
+            # will be in self._background_tasks now.  Re-check.
         self._background_tasks.clear()
         self._expected_cancelled_tasks.clear()
         self._pending_messages.clear()