Iceberg sink integration tests fail due to container readiness check matching stale Jetty log format

[krishvishal]

All three Iceberg sink integration tests fail with a container startup timeout:

• iceberg_sink_initializes_and_runs
• iceberg_sink_consumes_json_messages - iceberg_sink_handles_bulk_messages

   failed to setup fixture: FixtureSetup {
     fixture_type: "IcebergRestContainer",                                                                                                                                                   
     message: "Failed to start container: container is not ready: container startup timeout"
  }                                                                                                                                                                                         

Root cause: The IcebergRestContainer fixture uses WaitFor::message_on_stderr("Started Server@") to detect readiness. The apache/iceberg-rest-fixture:latest image upgraded to Jetty 12, which changed the startup log from Started Server@... to Started oejs.Server@.... The readiness check never matches, so the container times out after 30 seconds.