Generated unit tests in ads templates fail with `AttributeError: to_json`

[parthea]

In PR #1749, a new fragment test was added which uncovered an issue with the generated unit test for ads. This is related to #1596. There were certain places in the ads templates that were missed as part of #1596.

See build log here:
https://github.com/googleapis/gapic-generator-python/actions/runs/6026704981/job/16350267511?pr=1749

        # Mock the http request call within the method and fake a response.
        with mock.patch.object(type(client.transport._session), 'request') as req:
            # Designate an appropriate value for the returned response.
            return_value = service_pb2.Service(
                  name='name_value',
                  title='title_value',
                  producer_project_id='producer_project_id_value',
                  id='id_value',
            )
    
            # Wrap the value into a proper Response obj
            response_value = Response()
            response_value.status_code = 200
>           json_return_value = service_pb2.Service.to_json(return_value)
E           AttributeError: to_json

/tmp/tmpdvtl_8w9/tests/unit/gapic/fragment/test_service_manager.py:666: AttributeError
__________________ test_create_service_config_rest_flattened ___________________

    def test_create_service_config_rest_flattened():
        client = ServiceManagerClient(
            credentials=ga_credentials.AnonymousCredentials(),
            transport="rest",
        )
    
        # Mock the http request call within the method and fake a response.
        with mock.patch.object(type(client.transport._session), 'request') as req:
            # Designate an appropriate value for the returned response.
            return_value = service_pb2.Service()
    
            # Wrap the value into a proper Response obj
            response_value = Response()
            response_value.status_code = 200
>           json_return_value = service_pb2.Service.to_json(return_value)
E           AttributeError: to_json

/tmp/tmpdvtl_8w9/tests/unit/gapic/fragment/test_service_manager.py:832: AttributeError

The issue is the we're trying to call .to_json() on a protobuf message and .to_json() available for proto-plus messages.