agent_ui: Restrict agent toolbar width only for threads (#56410)

For all other modes, we want full-width view

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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A
cb715fb374Ben Brandt committed on 5/11/2026, 1:29:09 PM· committed by GitHubparent0efac16
1 file changedLine totals unavailable