Components.AI: Restore conversations to the last meaningful reading position

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

Restoring a conversation can land at the top or silently at the bottom of unseen content instead of where reading stopped.

### Describe the solution you'd like

Define and implement a meaningful restore anchor representing the end of previously read content, including refresh, rerender, and conversation-switch scenarios.

### Additional context

Leaf task for #68227 requirement B7. Builds on lifecycle notifications and the virtualized message list.

Start with parent issue #68227 requirement B7, then trace the lifecycle notifications and virtualized message list mentioned here. Define the restore anchor as the end of previously read content and verify it across refresh, rerender, and conversation-switch scenarios; done means restoration no longer jumps to the top or silently past unseen content.

Written by the indexing model from the issue text.