Components.AI: Validate long-conversation scenarios against Virtualize

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

The conversation UI needs variable-height streaming items, selectable follow behavior, position restoration, jump-to-item, and upward paging, but those scenarios have not been proven against the .NET 11 `Virtualize` implementation.

### Describe the solution you'd like

Build focused validation coverage for each scenario against the real component. Record supported behavior and file narrowly scoped framework issues for demonstrated gaps, especially height changes during streaming and prepend paging without viewport movement.

Do not add Components.AI-specific scrolling interop workarounds.

### Additional context

Leaf task for #68227 requirement B2.

Start with the real .NET 11 Virtualize component and validate variable-height streaming items, selectable follow behavior, position restoration, jump-to-item, and upward paging. Record which scenarios are supported, and file narrowly scoped framework issues for demonstrated gaps, especially streaming height changes and prepend paging without viewport movement.

Written by the indexing model from the issue text.