Code execution via unrestricted checkpoint object revival

Unsafe deserialization in LangGraph Checkpoint 4.1.0 and earlier allows attackers with checkpoint-store write access to execute code. The JsonPlusSerializer JSON revival path reconstructs Python objects from stored payloads without restricting revival to the expected constructor operation. Exploitation requires a persistent checkpointer, later checkpoint loading or resumption, and unauthorized modification of bytes at rest; code runs in the worker process.