chore: Defer import of google.cloud.storage in GCSArtifactService

The `google.cloud.storage` module is now imported within the `__init__` method of `GCSArtifactService`, rather than at the top level. This avoids importing the potentially heavy `storage` module unless an instance of `GCSArtifactService` is actually created.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 831124463

Author: wuliang229

src/google/adk/artifacts/gcs_artifact_service.py

@@ -27,7 +27,6 @@
 from typing import Any
 from typing import Optional
 
-from google.cloud import storage
 from google.genai import types
 from typing_extensions import override
 
@@ -47,6 +46,8 @@ def __init__(self, bucket_name: str, **kwargs):
         bucket_name: The name of the bucket to use.
         **kwargs: Keyword arguments to pass to the Google Cloud Storage client.
     """
+    from google.cloud import storage
+
     self.bucket_name = bucket_name
     self.storage_client = storage.Client(**kwargs)
     self.bucket = self.storage_client.bucket(self.bucket_name)