fix(checkpoint/serde): use lazy isinstance check for DiffDelta

Replace duck-typing check with lazy import inside _is_diff_delta helper
function to avoid module-level circular dependency while using proper
isinstance semantics.

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

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

@@ -47,6 +47,11 @@ EMPTY_BYTES = b""
 logger = logging.getLogger(__name__)
 
 
+def _is_diff_delta(obj: Any) -> bool:
+    from langgraph.checkpoint.base import DiffDelta  # lazy import avoids circular dep
+    return isinstance(obj, DiffDelta)
+
+
 class JsonPlusSerializer(SerializerProtocol):
     """Serializer that uses ormsgpack, with optional fallbacks.
 
@@ -239,11 +244,7 @@ class JsonPlusSerializer(SerializerProtocol):
             return "bytes", obj
         elif isinstance(obj, bytearray):
             return "bytearray", obj
-        elif (
-            type(obj).__name__ == "DiffDelta"
-            and hasattr(obj, "delta")
-            and hasattr(obj, "prev_version")
-        ):
+        elif _is_diff_delta(obj):
             return "diff", _msgpack_enc({"d": obj.delta, "p": obj.prev_version})
         else:
             try: