agent_ui: Treat whitespace-only input as empty in MessageEditor (#55530)
GitGlimpse895
created
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
Closes #54276
Release Notes:
- Fixed: the Send button in the Agent panel no longer activates when
the message input contains only whitespace (spaces or tabs).