Components.AI follow-up queue

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

Users cannot compose follow-up messages while an answer is streaming, and the engine has no queue semantics for atomically dispatching, removing, or cancelling pending follow-ups.

### Describe the solution you'd like

Track the design and implementation of the follow-up queue defined by D11 in #68227: unbounded pending messages, per-message removal, full-queue cancellation on Stop, and atomic drain into one next turn. Define ownership, editability, and attachment lifetime before implementation.

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

### Additional context

Covers C11 and open design decisions 7-8 from #68227.

Start with the D11 design and C11 context in parent issue #68227, including open design decisions 7-8. Define ownership, editability, and attachment lifetime for unbounded pending follow-ups, then document how per-message removal, Stop cancellation, and atomic draining should work. Done means the design is resolved and the independently mergeable sub-issues are clearly scoped.

Written by the indexing model from the issue text.