agent: Differentiate @mentions from markdown links (#28073)

This ensures that we display @mentions and normal markdown links
differently:

<img width="670" alt="Screenshot 2025-04-04 at 11 07 51"
src="https://github.com/user-attachments/assets/0a4d0881-abb9-42a8-b3fa-912cd6873ae0"
/>


Release Notes:

- N/A
1db3d92066Bennet Bo Fenner committed on 4/4/2025, 9:39:48 AM· committed by GitHubparenta7674d3
4 files changedLine totals unavailable