Now function WebAssert::responseContains does comparison in case-insensitive mode because of usage stripos php function:

And I see no ways to make case-sensitive comparison. So will be good to add option to enable case-sensitive comparison mode via some option. Or it is already exists, please point me out to the solution.

And will be good to explain that this function does case-sensitive comparison by default, because it is a surprise for many developers. For example, this nuance brings that issue in Drupal module: https://www.drupal.org/project/layout_builder_restrictions/issues/3278651