assistant2: Refine buffer inline assistant styles (#22377)

A lot of spacing tweaks. But, most notably, using the buffer font for
the inline assistant.

<img width="800" alt="Screenshot 2024-12-23 at 12 46 16 PM"
src="https://github.com/user-attachments/assets/ee2908a7-7515-4244-83fc-791172b29364"
/>

Release Notes:

- N/A