feat(checkpoint/serde): serialize DiffDelta as 'diff' type tag

Add serde support for DiffDelta by implementing dump/load for the "diff" type tag.
This allows the checkpoint system to efficiently store delta objects by serializing
them as msgpack-encoded dicts with {"d": delta, "p": prev_version} structure.

The implementation uses runtime type checking to avoid circular imports and
leverages the existing msgpack ext hooks for proper deserialization of complex
types like LangChain messages.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

libs/checkpoint/tests/test_jsonplus.py

@@ -983,3 +983,35 @@ def test_msgpack_nested_pydantic_serializes_as_dict(
     # No blocking should occur - inner is serialized as dict, not ext
     assert "blocked" not in caplog.text.lower()
     assert result == obj
+
+
+def test_diff_delta_serde_round_trip() -> None:
+    from langgraph.checkpoint.base import DiffDelta
+
+    serde = JsonPlusSerializer()
+    prev = "00000000000000000000000000000001.1234567890123456"
+    delta = DiffDelta(
+        delta=[HumanMessage(content="hello", id="msg-1")],
+        prev_version=prev,
+    )
+    type_tag, blob = serde.dumps_typed(delta)
+    assert type_tag == "diff"
+
+    result = serde.loads_typed(("diff", blob))
+    assert isinstance(result, dict)
+    assert result["p"] == prev
+    assert len(result["d"]) == 1
+    assert result["d"][0].content == "hello"
+
+
+def test_diff_delta_serde_root_blob() -> None:
+    from langgraph.checkpoint.base import DiffDelta
+
+    serde = JsonPlusSerializer()
+    delta = DiffDelta(delta=[], prev_version=None)
+    type_tag, blob = serde.dumps_typed(delta)
+    assert type_tag == "diff"
+
+    result = serde.loads_typed(("diff", blob))
+    assert result["p"] is None
+    assert result["d"] == []