fix: Add endpoints to get/list artifact version metadata

This change introduces new FastAPI endpoints in adk_web_server.py and corresponding client methods in adk_web_server_client.py to allow fetching metadata for artifact versions without downloading the artifact content

Close #3710

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 868217569

tests/unittests/cli/test_fast_api.py

@@ -290,6 +290,48 @@ def mock_artifact_service():
       })
       return version
 
+    def add_artifact(
+        self,
+        *,
+        app_name: str,
+        user_id: str,
+        session_id: str,
+        filename: str,
+        artifact: types.Part,
+        custom_metadata: Optional[dict[str, Any]] = None,
+        canonical_uri: Optional[str] = None,
+        mime_type: Optional[str] = None,
+    ) -> int:
+      """Synchronous helper for tests to add artifacts."""
+      key = _artifact_key(app_name, user_id, session_id, filename)
+      entries = artifacts.setdefault(key, [])
+      version = len(entries)
+      artifact_version = ArtifactVersion(
+          version=version,
+          canonical_uri=(
+              canonical_uri
+              or _canonical_uri(
+                  app_name, user_id, session_id, filename, version
+              )
+          ),
+          custom_metadata=custom_metadata or {},
+      )
+      if mime_type:
+        artifact_version.mime_type = mime_type
+      elif artifact.inline_data is not None:
+        artifact_version.mime_type = artifact.inline_data.mime_type
+      elif artifact.text is not None:
+        artifact_version.mime_type = "text/plain"
+      elif artifact.file_data is not None:
+        artifact_version.mime_type = artifact.file_data.mime_type
+
+      entries.append({
+          "version": version,
+          "artifact": artifact,
+          "metadata": artifact_version,
+      })
+      return version
+
     async def load_artifact(
         self, app_name, user_id, session_id, filename, version=None
     ):
@@ -322,6 +364,15 @@ def mock_artifact_service():
         return []
       return [entry["version"] for entry in artifacts[key]]
 
+    async def list_artifact_versions(
+        self, app_name, user_id, session_id, filename
+    ):
+      """List all artifact versions with metadata."""
+      key = _artifact_key(app_name, user_id, session_id, filename)
+      if key not in artifacts:
+        return []
+      return [entry["metadata"] for entry in artifacts[key]]
+
     async def delete_artifact(self, app_name, user_id, session_id, filename):
       """Delete an artifact."""
       key = _artifact_key(app_name, user_id, session_id, filename)
@@ -1193,6 +1244,69 @@ def test_save_artifact_returns_500_on_unexpected_error(
   assert response.json()["detail"] == "unexpected failure"
 
 
+def test_get_artifact_version_metadata(
+    test_app, create_test_session, mock_artifact_service
+):
+  """Test retrieving metadata for a specific artifact version."""
+  info = create_test_session
+  mock_artifact_service.add_artifact(
+      app_name=info["app_name"],
+      user_id=info["user_id"],
+      session_id=info["session_id"],
+      filename="report.txt",
+      artifact=types.Part(text="hello"),
+      custom_metadata={"foo": "bar"},
+      mime_type="text/plain",
+  )
+
+  url = (
+      f"/apps/{info['app_name']}/users/{info['user_id']}/sessions/"
+      f"{info['session_id']}/artifacts/report.txt/versions/0/metadata"
+  )
+  response = test_app.get(url)
+
+  assert response.status_code == 200
+  data = response.json()
+  assert data["version"] == 0
+  assert data["customMetadata"] == {"foo": "bar"}
+  assert data["mimeType"] == "text/plain"
+
+
+def test_list_artifact_versions_metadata(
+    test_app, create_test_session, mock_artifact_service
+):
+  """Test listing metadata for all versions of an artifact."""
+  info = create_test_session
+  mock_artifact_service.add_artifact(
+      app_name=info["app_name"],
+      user_id=info["user_id"],
+      session_id=info["session_id"],
+      filename="report.txt",
+      artifact=types.Part(text="v0"),
+  )
+  mock_artifact_service.add_artifact(
+      app_name=info["app_name"],
+      user_id=info["user_id"],
+      session_id=info["session_id"],
+      filename="report.txt",
+      artifact=types.Part(text="v1"),
+      custom_metadata={"foo": "bar"},
+  )
+
+  url = (
+      f"/apps/{info['app_name']}/users/{info['user_id']}/sessions/"
+      f"{info['session_id']}/artifacts/report.txt/versions/metadata"
+  )
+  response = test_app.get(url)
+
+  assert response.status_code == 200
+  data = response.json()
+  assert isinstance(data, list)
+  assert len(data) == 2
+  assert data[1]["version"] == 1
+  assert data[1]["customMetadata"] == {"foo": "bar"}
+
+
 def test_get_eval_set_result_not_found(test_app):
   """Test getting an eval set result that doesn't exist."""
   url = "/apps/test_app_name/eval_results/test_eval_result_id_not_found"