Improves upon https://github.com/zed-industries/zed/pull/42674

Before:

<img width="520" height="202" alt="image"
src="https://github.com/user-attachments/assets/efb1650b-4c0e-4424-8d9b-90de80c72df2"
/> <img width="157" height="211" alt="image"
src="https://github.com/user-attachments/assets/cf4605f3-88e5-4724-ad2b-1219ed04a945"
/>

After:

<img width="529" height="208" alt="image"
src="https://github.com/user-attachments/assets/382fd523-a3d9-4700-a8df-c339419fc6dc"
/>
<img width="133" height="208" alt="image"
src="https://github.com/user-attachments/assets/f22b72d9-d416-47f9-92af-ea1de6fb5583"
/>



Release Notes:

- Fixed an issue where Markdown tables would sometimes show double
borders.