fix: Access actual client from holder in flush worker

The flush worker was calling _flush() on _TelemetryClientHolder objects
instead of the actual TelemetryClient. Fixed by accessing holder.client
before calling _flush().

Fixes AttributeError in e2e tests: '_TelemetryClientHolder' object has
no attribute '_flush'

Author: samikshya-db

src/databricks/sql/telemetry/telemetry_client.py

@@ -504,8 +504,8 @@ def _flush_worker(cls):
             with cls._lock:
                 clients_to_flush = list(cls._clients.values())
 
-                for client in clients_to_flush:
-                    client._flush()
+                for holder in clients_to_flush:
+                    holder.client._flush()
 
     @classmethod
     def _stop_flush_thread(cls):