chore: rename serde type tag "diff" → "delta" for DeltaValue

Consistent with channel/type naming (DeltaChannel, DeltaValue).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py

@@ -246,7 +246,7 @@ class JsonPlusSerializer(SerializerProtocol):
         elif isinstance(obj, bytearray):
             return "bytearray", obj
         elif _is_delta_value(obj):
-            return "diff", _msgpack_enc({"d": obj.delta, "c": obj.prev_checkpoint_id})
+            return "delta", _msgpack_enc({"d": obj.delta, "c": obj.prev_checkpoint_id})
         else:
             try:
                 return "msgpack", _msgpack_enc(obj)
@@ -269,7 +269,7 @@ class JsonPlusSerializer(SerializerProtocol):
             return ormsgpack.unpackb(
                 data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
             )
-        elif type_ == "diff":
+        elif type_ == "delta":
             from langgraph.checkpoint.base import DeltaValue  # lazy import
 
             raw = ormsgpack.unpackb(

libs/checkpoint/tests/test_jsonplus.py

@@ -994,7 +994,7 @@ def test_delta_value_serde_round_trip() -> None:
         delta=[{"type": "human", "content": "hi"}], prev_checkpoint_id="abc-123"
     )
     type_tag, blob = serde.dumps_typed(original)
-    assert type_tag == "diff"
+    assert type_tag == "delta"
     loaded = serde.loads_typed((type_tag, blob))
     assert isinstance(loaded, DeltaValue)
     assert loaded.delta == original.delta