Components.AI: Implement atomic follow-up queue state and dispatch

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

The engine cannot accept follow-up messages while streaming or atomically dispatch a pending batch as one turn.

### Describe the solution you'd like

Implement the approved queue model with unbounded ordered entries, per-entry removal, composer clearing after enqueue, full-queue cancellation on Stop, and atomic drain of all pending messages into one next turn after normal completion.

### Additional context

Leaf task for #68227 requirement C11. Depends on queue semantics and lifecycle APIs.

Start with requirement C11 in #68227, then read the existing queue semantics and lifecycle APIs that this leaf task depends on. Verify the implementation supports ordered unbounded entries, per-entry removal, composer clearing after enqueue, full cancellation on Stop, and atomic draining into one next turn after normal completion.

Written by the indexing model from the issue text.