assistant2: Add spacing between historical thread entries (#22140)

This PR adds spacing between entries in the historical threads view:

<img width="1371" alt="Screenshot 2024-12-17 at 10 51 26 AM"
src="https://github.com/user-attachments/assets/f66d69e9-eb59-4c76-b3b5-c4d60190c3cc"
/>

Release Notes:

- N/A