fix: Support saving text artifacts in GCS artifact service

Resolves https://github.com/google/adk-python/issues/2775

PiperOrigin-RevId: 802850139

src/google/adk/artifacts/gcs_artifact_service.py

@@ -180,10 +180,17 @@ class GcsArtifactService(BaseArtifactService):
     )
     blob = self.bucket.blob(blob_name)
 
-    blob.upload_from_string(
-        data=artifact.inline_data.data,
-        content_type=artifact.inline_data.mime_type,
-    )
+    if artifact.inline_data:
+      blob.upload_from_string(
+          data=artifact.inline_data.data,
+          content_type=artifact.inline_data.mime_type,
+      )
+    elif artifact.text:
+      blob.upload_from_string(
+          data=artifact.text,
+      )
+    else:
+      raise ValueError("Artifact must have either inline_data or text.")
 
     return version
 

tests/unittests/artifacts/test_artifact_service.py

@@ -258,8 +258,9 @@ async def test_list_versions(service_type):
       )
       for i in range(3)
   ]
+  versions.append(types.Part.from_text(text="hello"))
 
-  for i in range(3):
+  for i in range(4):
     await artifact_service.save_artifact(
         app_name=app_name,
         user_id=user_id,
@@ -275,4 +276,4 @@ async def test_list_versions(service_type):
       filename=filename,
   )
 
-  assert response_versions == list(range(3))
+  assert response_versions == list(range(4))