fix: preserve empty GCS text artifacts

Merge google#5724

## Summary

- allow `GcsArtifactService` to save `Part(text="")` as a valid text artifact
- load GCS blobs via `get_blob()` so missing objects are distinct from zero-byte objects
- add regression coverage for saving and loading an empty GCS text artifact

## Context

The file artifact backend already persists empty text artifacts, but the GCS backend checked `artifact.text` by truthiness and rejected empty strings as missing payloads. It also treated `download_as_bytes() == b""` as a missing artifact, which collapses a valid zero-byte GCS object into `None`.

This keeps the existing GCS retrieval shape for text artifacts: they load as `inline_data` with `text/plain`, matching current non-empty text behavior.

## Validation

- `uv run --extra test pytest tests/unittests/artifacts/test_artifact_service.py -q` -> 60 passed
- `uv run --extra dev pyink --check src/google/adk/artifacts/gcs_artifact_service.py tests/unittests/artifacts/test_artifact_service.py` -> passed
- `uv run --extra dev isort --check-only src/google/adk/artifacts/gcs_artifact_service.py tests/unittests/artifacts/test_artifact_service.py` -> passed
- `python3 -m py_compile src/google/adk/artifacts/gcs_artifact_service.py tests/unittests/artifacts/test_artifact_service.py` -> passed
- `git diff --check` -> passed

Co-authored-by: Bo Yang <ybo@google.com>
COPYBARA_INTEGRATE_REVIEW=google#5724 from pragnyanramtha:pragnyan/gcs-empty-text-artifacts 66aba44
PiperOrigin-RevId: 932789425

Author: 2 people

src/google/adk/artifacts/gcs_artifact_service.py

@@ -224,7 +224,7 @@ def _save_artifact(
           data=artifact.inline_data.data,
           content_type=artifact.inline_data.mime_type,
       )
-    elif artifact.text:
+    elif artifact.text is not None:
       blob.upload_from_string(
           data=artifact.text,
           content_type="text/plain",
@@ -263,11 +263,11 @@ def _load_artifact(
     blob_name = self._get_blob_name(
         app_name, user_id, filename, version, session_id
     )
-    blob = self.bucket.blob(blob_name)
+    blob = self.bucket.get_blob(blob_name)
+    if not blob:
+      return None
 
     artifact_bytes = blob.download_as_bytes()
-    if not artifact_bytes:
-      return None
     artifact = types.Part.from_bytes(
         data=artifact_bytes, mime_type=blob.content_type
     )

tests/unittests/artifacts/test_artifact_service.py

@@ -644,6 +644,35 @@ async def test_get_artifact_version_out_of_index(
   )
 
 
+@pytest.mark.asyncio
+async def test_gcs_save_and_load_empty_text_artifact(
+    artifact_service_factory,
+):
+  """GcsArtifactService should treat empty text as stored content."""
+  artifact_service = artifact_service_factory(ArtifactServiceType.GCS)
+  artifact = types.Part.from_text(text="")
+
+  version = await artifact_service.save_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="123",
+      filename="empty.txt",
+      artifact=artifact,
+  )
+
+  assert version == 0
+  loaded_artifact = await artifact_service.load_artifact(
+      app_name="app0",
+      user_id="user0",
+      session_id="123",
+      filename="empty.txt",
+  )
+
+  assert loaded_artifact == types.Part.from_bytes(
+      data=b"", mime_type="text/plain"
+  )
+
+
 @pytest.mark.asyncio
 async def test_file_metadata_camelcase(tmp_path, artifact_service_factory):
   """Ensures FileArtifactService writes camelCase metadata without newlines."""