Components.AI: Virtualize the conversation message list

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

`MessageList` eagerly renders the full conversation and cannot scale to long histories.

### Describe the solution you'd like

Adopt `Virtualize` for stable, variable-height turns that can grow while streaming. Support load-older behavior and preserve the visible reading position. Consume framework fixes identified by the validation task rather than adding local scrolling interop.

### Additional context

Leaf task for #68227 requirement B1. Depends on stable identities and Virtualize scenario validation.

Start with the Components.AI MessageList and the Virtualize scenario validation associated with #68227 requirement B1. Confirm how stable identities, variable-height streaming turns, load-older behavior, and visible reading position are expected to work; done means the full conversation is virtualized and the validation task passes without local scrolling interop.

Written by the indexing model from the issue text.