Render the first character as the icon in a terminal thread (#58251)

This makes terminal agents look much better:

<img width="667" height="520" alt="Screenshot 2026-06-01 at 10 35 34 AM"
src="https://github.com/user-attachments/assets/9029ca7d-effe-43e5-b8f4-10b3e42e6cc9"
/>

<img width="767" height="579" alt="Screenshot 2026-06-01 at 10 36 23 AM"
src="https://github.com/user-attachments/assets/d3392f64-ca2c-40d2-9077-b2ac7a51151d"
/>

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- The first icon in the terminal title will now render in the thread
icon slot, when using terminal threads