Python: sync AG-UI conversation history from backend

For now, the Agent state is passed from the frontend. But the frontend is not persistent. So in an ideal scenario backend stores the state and syncs it via SNAPSHOT in the initial connection. In such a way, it will be possible to refresh the web browser page and see the whole history.

LangGraph is already done this feature and stores a graph as a global object.

I'll pick this up in the next day or two. Apologies for the delay.