tests: fixing flaky test: test_create_job_configs_labels_log_adaptor_call_method_under_length_limit

[chelsea-lin]

This change addresses flakiness in the test_create_job_configs_labels_log_adaptor_call_method_under_length_limit test. The test occasionally failed because a session-__del__ label was unexpectedly included in the logged adaptor call methods, as shown in the diff below:

  Full diff:
    {
     'bigframes-api': 'dataframe-columns',
     'recent-bigframes-api-0': 'dataframe-max',
     'recent-bigframes-api-1': 'dataframe-head',
     'recent-bigframes-api-2': 'dataframe-__init__',
  +  'recent-bigframes-api-3': 'session-__del__',
     'source': 'bigquery-dataframes-temp',
    }