Add public API tests for `GenerateContentResponse` in Vertex AI #4982

Job	Run time
update_SpecsTesting_repo	32s
specs_checking	0s
buildup_SpecsTesting_repo_FirebaseCore	0s
buildup_SpecsTesting_repo	0s
abtesting_quickstart	0s
auth_quickstart	0s
crashlytics_quickstart	0s
database_quickstart	0s
dynamiclinks_quickstart	0s
firestore_quickstart	0s
functions_quickstart	0s
messaging_quickstart	0s
inappmessaging_quickstart	0s
remoteconfig_quickstart	0s
storage_quickstart	0s
performance_quickstart	0s
	32s

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