agent_ui: Fix full-width thread view layout (#56684)

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

Closes #56677

Release Notes:

- Agent: Fix squashed message editor when `limit_content_width=false`

crates/agent_ui/src/conversation_view/thread_view.rs

@@ -3300,6 +3300,7 @@ impl ThreadView {
             .child(
                 v_flex()
                     .when_some(max_content_width, |this, max_w| this.flex_basis(max_w))
+                    .when(max_content_width.is_none(), |this| this.w_full())
                     .when(fills_container, |this| this.h_full())
                     .flex_shrink()
                     .flex_grow_0()