assistant: Use code label for tab slash command completions (#17296)

This adopts the same approach we use for the `/file` command, which has
the benefit that, even if the path is long, the filename is always
visible.

| Before | After |
|--------|-------|
| <img width="564" alt="image"
src="https://github.com/user-attachments/assets/a43574af-e4c1-4f11-be70-49d6020557c4">
| <img width="567" alt="image"
src="https://github.com/user-attachments/assets/4db383b9-5039-4f35-b821-e1cc1a4ea7e8">
|


Release Notes:

- Improved UX of tab slash command completions