Improve UI of popover buttons when hovering over chat messages (#9041)

### Before


https://github.com/zed-industries/zed/assets/146845123/4a16c1ce-a671-4e39-abc9-3a0cb25bc0cd

### After


https://github.com/zed-industries/zed/assets/146845123/cfab3d00-246e-427d-9c40-8ee520a0a186




Release Notes:
- Improved the UI of popover buttons when hovering over chat messages.