This update modifies the initialization of the left-hand side
multibuffer in the SplittableEditor. It now checks if the right-hand
side multibuffer is a singleton and uses a
`MultiBuffer::without_headers` instead.


Before Screenshot:
<img width="1624" height="1030" alt="Screenshot 2026-05-07 at 7 30
16 PM"
src="https://github.com/user-attachments/assets/3d963703-309c-42e4-b2be-fe64bd9c0a06"
/>

After Screenshot:
<img width="1624" height="1030" alt="Screenshot 2026-05-07 at 7 32
48 PM"
src="https://github.com/user-attachments/assets/51668319-6a34-47df-b8b1-8bf58b86407e"
/>


Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)



Release Notes:
- Optimized multibuffer creation by conditionally using headers based on
RHS state.