No separating line in summary with hidden column

See the summary in the Hidden columns section [here](https://diffly.readthedocs.io/stable/guides/features/summary.html#hidden-columns): there should be a horizontal line between `loyalty_card_id` and `product`

<img width="722" height="481" alt="Image" src="https://github.com/user-attachments/assets/224de168-0de6-4655-a966-48c4d9fc3c8c" />