fix: restore .text attribute when loading text artifacts from GCS

GcsArtifactService._load_artifact() always used Part.from_bytes()
which populates inline_data even for text/plain content. This caused
Part.from_text() artifacts to lose their .text attribute after a
save/load cycle.

Now detects text/plain content type and returns Part(text=...) instead,
matching the behavior of FileArtifactService which already handles
this correctly.

Fixes #3157

Author: Maanik23

src/google/adk/artifacts/gcs_artifact_service.py

@@ -268,6 +268,10 @@ def _load_artifact(
     artifact_bytes = blob.download_as_bytes()
     if not artifact_bytes:
       return None
+    # Restore text artifacts with the .text attribute rather than
+    # inline_data so that round-tripping Part.from_text() works correctly.
+    if blob.content_type and blob.content_type.startswith("text/plain"):
+      return types.Part(text=artifact_bytes.decode("utf-8"))
     artifact = types.Part.from_bytes(
         data=artifact_bytes, mime_type=blob.content_type
     )

tests/unittests/artifacts/test_artifact_service.py

@@ -896,3 +896,49 @@ async def test_save_artifact_with_snake_case_dict(
   assert loaded is not None
   assert loaded.inline_data is not None
   assert loaded.inline_data.mime_type == "text/plain"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "service_type",
+    [
+        ArtifactServiceType.IN_MEMORY,
+        ArtifactServiceType.GCS,
+        ArtifactServiceType.FILE,
+    ],
+)
+async def test_text_artifact_roundtrip(service_type, artifact_service_factory):
+  """Text artifacts saved via Part.from_text() should preserve .text on load.
+
+  Regression test for https://github.com/google/adk-python/issues/3157
+  """
+  artifact_service = artifact_service_factory(service_type)
+  app_name = "app0"
+  user_id = "user0"
+  session_id = "sess0"
+  filename = "report.txt"
+  text_content = '{"status": "success", "report": "Sunny, 25C"}'
+
+  artifact = types.Part.from_text(text=text_content)
+  assert artifact.text == text_content  # sanity check
+
+  version = await artifact_service.save_artifact(
+      app_name=app_name,
+      user_id=user_id,
+      session_id=session_id,
+      filename=filename,
+      artifact=artifact,
+  )
+  assert version == 0
+
+  loaded = await artifact_service.load_artifact(
+      app_name=app_name,
+      user_id=user_id,
+      session_id=session_id,
+      filename=filename,
+  )
+  assert loaded is not None
+  assert loaded.text == text_content, (
+      f"Expected .text='{text_content}', got .text={loaded.text!r} "
+      f"(inline_data={loaded.inline_data!r})"
+  )