Components.AI long-conversation support

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

`MessageList` eagerly renders every turn and has no robust follow, restore, jump-to-latest, or upward-paging behavior for long and streaming conversations.

### Describe the solution you'd like

Track validation and implementation of long-conversation behavior using the framework `Virtualize` component, including dynamic-height streaming content, selectable follow behavior, position restoration, upward paging, and per-message semantics.

Any demonstrated `Virtualize` gaps should be filed as focused framework tasks rather than worked around locally.

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

### Additional context

Covers requirements B1-B7 and I2 from #68227.

Start with the parent workstream in #68227 and its native sub-issues, then inspect the Components.AI MessageList and Virtualize behavior. Validate requirements B1-B7 and I2, including streaming height, follow and restore behavior, jump-to-latest, upward paging, and per-message semantics; any Virtualize gaps should become separate framework tasks.

Written by the indexing model from the issue text.