fix(a2a): register ResponseFormat in checkpoint serializer allowlist

Configure MemorySaver with a JsonPlusSerializer that explicitly allows
deserialization of app.agent.ResponseFormat from checkpoints. This
silences the langgraph warning about deserializing unregistered types,
which will become a hard error in a future version.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Paolo Dettori <dettori@us.ibm.com>

Author: pdettori

a2a/a2a_currency_converter/app/agent.py

@@ -9,11 +9,16 @@
 from langchain_core.tools import tool
 from langchain_openai import ChatOpenAI
 from langgraph.checkpoint.memory import MemorySaver
+from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
 from langgraph.prebuilt import create_react_agent
 from pydantic import BaseModel
 from pydantic_settings import BaseSettings
 
-memory = MemorySaver()
+memory = MemorySaver(
+    serde=JsonPlusSerializer(
+        allowed_msgpack_modules=[("app.agent", "ResponseFormat")],
+    ),
+)
 
 
 class Configuration(BaseSettings):

tests/a2a/test_currency_converter.py

@@ -15,6 +15,8 @@
     "langgraph",
     "langgraph.checkpoint",
     "langgraph.checkpoint.memory",
+    "langgraph.checkpoint.serde",
+    "langgraph.checkpoint.serde.jsonplus",
     "langgraph.prebuilt",
 ]:
     sys.modules.setdefault(mod, MagicMock())