assistant: Preserve selection focus in the model selector (#23713)

This PR fixes an incorrect behavior in the model selector where we were
removing the focus from the selected item back to the very first item of
the list. Now, if you click/hit return on an item, focus is preserved
there. In other words, focus is always initially in the selected item.

### Before

https://github.com/user-attachments/assets/62b72b1f-4e32-4b4a-adff-dcf9a2c13a28

### After

https://github.com/user-attachments/assets/a8528933-da01-481a-96f3-0173a39a03c0

Release Notes:

- N/A