Evaluate to use `contain-intrinsic-size` for test pages

couldn't we use [`contain-intrinsic-size`](https://developer.mozilla.org/en-US/docs/Web/CSS/contain-intrinsic-size) for those test pages?

_Originally posted by @mfranzke in https://github.com/db-ux-design-system/core-web/pull/4244#discussion_r2128560247_
            