assistant2: Add tool selector (#26480)

This PR adds a tool selector to Assistant 2 to facilitate customizing
the tools that the model sees:

<img width="1297" alt="Screenshot 2025-03-11 at 4 25 31 PM"
src="https://github.com/user-attachments/assets/7a656343-83bc-4546-9430-6a5f7ff1fd08"
/>

Release Notes:

- N/A