Always use mock GCS client in unit test.
Always use mock GCS client in unit test.

PiperOrigin-RevId: 336311465
Change-Id: Icde22b256c765354857902755b54e30b0bf412d7

GoogleCloudPlatform/healthcare-data-harmonization,GoogleCloudPlatform/healthcare-data-harmonization,GoogleCloudPlatform/healthcare-data-harmonization,GoogleCloudPlatform/healthcare-data-harmonization,GoogleCloudPlatform/healthcare-data-harmonization,GoogleCloudPlatform/healthcare-data-harmonization