assistant2: Fix toolbar layout shift (#22770)

Note how, previously, switching between the thread view and the history
caused a slightly reduction of the toolbar height. Super subtle stuff,
but doesn't happen anymore.

### Before


https://github.com/user-attachments/assets/712ff34e-a638-484d-8415-16011b10ae63

### After


https://github.com/user-attachments/assets/7ccff7a3-45a4-445c-9638-8445733e0ffc

Release Notes:

- N/A