Make sure list_manifest_layers is not callable for manifest lists

data/registry_model/shared.py

@@ -345,6 +345,8 @@ class SharedModel:
         working towards the leaf, including the associated Blob and its placements (if specified).
         Returns None if the manifest could not be parsed and validated.
     """
+    assert not parsed.is_manifest_list
+
     retriever = RepositoryContentRetriever(repo_id, storage)
     requires_empty_blob = parsed.get_requires_empty_layer_blob(retriever)