diagnostics use error severity in preview mode

With 2026.36.0, it differentiated yellow/red:
<img width="528" height="193" alt="Image" src="https://github.com/user-attachments/assets/2a7edfcd-04da-4c71-9fdb-c884ce57924a" />

Now it doesn't; only red:
<img width="519" height="193" alt="Image" src="https://github.com/user-attachments/assets/e7f38cfd-7169-47e0-a92a-48582637d5d9" />
