Add CookieJar serialization, deserialization, and cloning

Add CookieJar serialization, deserialization, and cloning: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). Right now there's no supported way to persist a CookieJar and load it back later. We need the ability to take a jar, turn it into a plain JSON-friendly structure, and…