Components.AI: Add AgentContext lifecycle and mutation APIs

### Is your feature request related to a problem? Please describe the problem. `AgentContext` only appends turns. It has no clear, removal, replacement, mutation notification, `CanSend`, or `IsBusy` contract, which blocks switching, retry, editing, queueing, and reliable restore. ### Describe the solution you'd like Design and add the minimum public lifecycle API needed to clear, remove, replace, and notify mutations while exposing authoritative interaction state. Include correct restore notifications and status transitions. ### Additional context Leaf task for #68227 requirements G6 and L2. Requires public API review.