Remove `AgentSession._chat_ctx` for simplicity

[wdhwg001]

Feature Type

Would make my life easier

Feature Description

It's currently exposed as AgentSession.history, but it's nothing but just a backlog, and the actual chat_ctx is the one in Agent.

Having two chat_ctx is kinda confusing, and there's no way to do update_chat_ctx to that either, so if we do advanced context engineering, AgentSession._chat_ctx will lose sync - despite it's already loosely synced with the Agent._chat_ctx.

It'd be really nice that we remove it completely to keep our life easier.

Thanks!

Workarounds / Alternatives

none - just don't use AgentSession.history.

Additional Context

No response