fix(gateway): drain pending messages via fresh task, not recursion (#17758)

`_process_message_background` finished a turn, found a queued
follow-up, and drained it via `await
self._process_message_background(pending_event, session_key)`.  Each
chained follow-up added a frame to the call stack instead of starting
fresh.  Under sustained pending-queue activity (e.g. a user sending
follow-ups faster than the agent finishes turns) the C stack would
exhaust at ~2000 nested frames and SIGSEGV the process.

Mirror the late-arrival drain pattern that already exists in the same
function: spawn a new `asyncio.create_task(...)` for the pending event
and return so the current frame can unwind.  The new task takes
ownership via `_session_tasks[session_key]`.

The late-arrival drain in `finally` could now race with the in-band
drain across the `await typing_task` / `await stop_typing` window, so
add a guard: if `_session_tasks[session_key]` is no longer the current
task, an in-band drain already spawned a follow-up task — re-queue the
late-arrival event so that task picks it up after its current event,
instead of spawning a second concurrent task for the same session_key.

Regression test (`test_pending_drain_no_recursion.py`) chains 12
follow-ups and asserts the recorded
`_process_message_background` stack depth stays bounded at handler
entry.  Pre-fix: depths grow linearly `[1,2,3,…,12]`.  Post-fix: all
depths are `1`.

`test_duplicate_reply_suppression::test_stale_response_suppressed_when_interrupted`
called `_process_message_background` directly and implicitly relied on
the old recursive `await` semantic — updated to wait for the spawned
drain task before checking the sent list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/gateway/test_duplicate_reply_suppression.py

@@ -108,6 +108,15 @@ class TestBaseInterruptSuppression:
 
         await adapter._process_message_background(event_a, session_key)
 
+        # The in-band pending-drain now hands off to a fresh task instead
+        # of recursing (#17758).  Wait for that task to finish before
+        # checking the sent list.
+        for _ in range(200):
+            if any(s["content"] == pending_response for s in adapter.sent):
+                break
+            await asyncio.sleep(0.01)
+        await adapter.cancel_background_tasks()
+
         # The stale response should NOT have been sent.
         stale_sends = [s for s in adapter.sent if s["content"] == stale_response]
         assert len(stale_sends) == 0, (