fix: handle ReferenceError in BigQueryAgentAnalyticsPlugin atexit cleanup

- Add try/except to catch ReferenceError when weakref proxy object is already garbage collected
- Prevents CI failure due to unhandled exception during test cleanup

Author: donggyun112

src/google/adk/plugins/bigquery_agent_analytics_plugin.py

@@ -1456,7 +1456,13 @@ def get_credentials():
   def _atexit_cleanup(batch_processor: "BatchProcessor") -> None:
     """Clean up batch processor on script exit."""
     # Check if the batch_processor object is still alive
-    if batch_processor and not batch_processor._shutdown:
+    # weakref.proxy raises ReferenceError if the referent has been garbage collected
+    try:
+      is_alive = batch_processor and not batch_processor._shutdown
+    except ReferenceError:
+      # Object has already been garbage collected, nothing to clean up
+      return
+    if is_alive:
       # Emergency Flush: Rescue any logs remaining in the queue
       remaining_items = []
       try: