Address Greptile review feedback

- SerializerStore now extends ABCMeta (not type) so @AbstractMethod
  is enforced — incomplete subclasses raise TypeError at definition
- Validate blobs non-empty before accessing blobs[0] in save_artifacts
- Add NOTE on compress_method not yet wired into save path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Saeid Barati

metaflow/datastore/artifacts/serializer.py

@@ -1,4 +1,4 @@
-from abc import abstractmethod
+from abc import ABCMeta, abstractmethod
 from collections import namedtuple
 
 
@@ -23,6 +23,9 @@ class SerializedBlob(object):
         If None, auto-detected from value type: str -> reference, bytes -> new data.
     compress_method : str
         Compression method for new blobs. Ignored for references. Default "gzip".
+        NOTE: Not yet wired into the save path — ContentAddressedStore currently
+        always applies gzip. This field is forward-looking for when per-blob
+        compression control is needed (e.g., multi-blob IOType support).
     """
 
     def __init__(self, value, is_reference=None, compress_method="gzip"):
@@ -43,7 +46,7 @@ def needs_save(self):
         return not self.is_reference
 
 
-class SerializerStore(type):
+class SerializerStore(ABCMeta):
     """
     Metaclass for ArtifactSerializer that auto-registers subclasses by TYPE.
 

metaflow/datastore/task_datastore.py

@@ -372,6 +372,11 @@ def serialize_iter():
 
                 # For now, serializers produce a single blob per artifact.
                 # Multi-blob support will be added when IOType lands.
+                if not blobs:
+                    raise DataException(
+                        "Serializer %s returned no blobs for artifact '%s'"
+                        % (serializer.__name__, name)
+                    )
                 artifact_names.append(name)
                 yield blobs[0].value