Long model identifiers (e.g., AWS Bedrock ARNs) overflow the bottom
toolbar, breaking the layout. Truncate display names to 32 characters
with an ellipsis when they exceed 33 characters.

### Before

<img width="1128" height="308" alt="after"
src="https://github.com/user-attachments/assets/000d5959-3bd5-4349-8f4e-09967ea7b2d8"
/>

### After

<img width="1010" height="212" alt="before"
src="https://github.com/user-attachments/assets/358fff8c-3bce-43ac-aed4-8ef630fc327b"
/>

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] 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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Agent: Improved truncation of long configuration button labels for
external agents.

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>