Now the @ completion menu items font size respect/match the buffer's
font size, as opposed to being rendered a bit bigger.

| Before | After |
|--------|--------|
| <img width="1226" height="468" alt="Screenshot 2025-09-02 at 11 
09@2x"
src="https://github.com/user-attachments/assets/a6d37110-b544-40c3-bf7a-447ea003d4d7"
/> | <img width="1218" height="462" alt="Screenshot 2025-09-02 at 11  09
2@2x"
src="https://github.com/user-attachments/assets/19e58bf8-2db5-442e-8f60-02dd9ee1308f"
/> |

Release Notes:

- inline assistant: Improved @-mention menu item font size, better
matching the buffer's font size.