test(diagnostics): add subscriber_dropped_events to diagnostic_snapshot exact-dict assertion

Co-authored-by: rh-id <rh-id@users.noreply.github.com>

tests/test_webui_runtime_diagnostics.py

@@ -10,6 +10,7 @@ def test_stream_channel_exposes_buffer_and_subscriber_counts():
         "subscriber_count": 0,
         "offline_buffered_events": 1,
         "offline_dropped_events": 0,
+        "subscriber_dropped_events": 0,
     }
 
     subscriber = channel.subscribe()