Closes #29532

Fixes case where the markdown background was not rendering correctly.
This regression was introduced in
https://github.com/zed-industries/zed/pull/26454.

<img
src="https://github.com/user-attachments/assets/1e64930f-c98e-4042-a20e-46eed03293d6"
alt="image" width="400" />


Release Notes:

- Fixed an issue where markdown code blocks did not wrap correctly.