assistant2: Add check icon for included context (#22774)

Quick follow-up to: https://github.com/zed-industries/zed/pull/22712 —
just to make it more visually easier to understand.

<img width="800" alt="Screenshot 2025-01-07 at 11 48 06 AM"
src="https://github.com/user-attachments/assets/92f0523b-eb85-4929-a825-2e1e524b3ad7"
/>

Release Notes:

- N/A