test(photon): drain detached fatal-notification task in zombie watchdog test

The lifecycle cluster made fatal notifications detached; the watchdog test
still asserted synchronously.

tests/plugins/platforms/photon/test_zombie_stream_watchdog.py

@@ -23,6 +23,7 @@ gRPC traffic occurs.
 """
 from __future__ import annotations
 
+import asyncio
 import json
 import subprocess
 from pathlib import Path
@@ -240,6 +241,13 @@ async def test_monitor_still_raises_fatal_when_zombie_degrades_stream(
 
     await adapter._monitor_sidecar_health()
 
+    # Fatal notification is dispatched from a detached task (so callers can't
+    # cancel their own handoff) — drain pending tasks before asserting.
+    for _ in range(50):
+        if notified:
+            break
+        await asyncio.sleep(0)
+
     assert adapter.has_fatal_error is True
     assert adapter.fatal_error_code == "UPSTREAM_STREAM_DEGRADED"
     assert adapter.fatal_error_retryable is True