Components.AI: Announce streaming responses without per-token screen-reader spam

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

Incoming assistant content is not announced; announcing every token would be unusable.

### Describe the solution you'd like

Design and implement a throttled or chunked live-region strategy that announces meaningful streaming updates, completion, stopping, and failure without moving focus or flooding assistive technology.

### Additional context

Leaf task for #68227 requirement I1.

Start with parent issue #68227 and requirement I1, then locate the Components.AI entry point responsible for streaming responses. Define completion as meaningful announcements for streaming updates, completion, stopping, and failure without focus movement or per-token screen-reader spam; inspect the component's existing tests before adding coverage.

Written by the indexing model from the issue text.