Components.AI conversation model and rendering foundations

### Is your feature request related to a problem? Please describe the problem.

The Components.AI UI currently lacks stable turn/message/block identity, models authorship at the wrong level, re-renders the full conversation while streaming, and lacks lifecycle APIs needed by several higher-level experiences.

### Describe the solution you'd like

Track the foundational work to:

- Correct turn and message semantics and assign stable identities.
- Scope streaming renders to the changing content.
- Add explicit `AgentContext` lifecycle and mutation APIs.
- Define consistent interaction-state and error behavior.
- Represent stopped and incomplete answers explicitly.

This is a workstream under #68227. Its independently mergeable tasks are represented as native sub-issues.

### Additional context

Covers requirements A2, A3, A7, F12, G2-G8, and L2 from #68227.

Start with the requirements in #68227 and identify the native sub-issues linked from this workstream. Work on one independently mergeable sub-issue; completion means implementing that sub-issue's defined foundation work rather than the entire conversation-model initiative.

Written by the indexing model from the issue text.