assistant2: Add thread history (#21599)

This PR adds support for thread history to the Assistant 2 panel.

We also now generate summaries for the threads.

<img width="986" alt="Screenshot 2024-12-05 at 12 56 53 PM"
src="https://github.com/user-attachments/assets/46cb1309-38a2-4ab9-9fcc-c1275d4b5f2c">

<img width="986" alt="Screenshot 2024-12-05 at 12 56 58 PM"
src="https://github.com/user-attachments/assets/8c91ba57-a6c5-4b88-be05-b22fb615ece5">

Release Notes:

- N/A

---------

Co-authored-by: Piotr <piotr@zed.dev>