- Stop merging same row diagnostics
- (for Rust) show code fragments surrounded by `'s in monospace

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>

Closes #29362

Release Notes:

- diagnostics: Diagnostics are no longer merged when they're on the same
line
- rust: Diagnostics now show code snippets in monospace font:

<img width="551" alt="Screenshot 2025-04-29 at 16 13 45"
src="https://github.com/user-attachments/assets/d289be31-717d-404f-a76a-a0cda3e96fbe"
/>

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>