Fix is_sorted Property Test to Call Sort Function

[valerio-iachini]

This updates the is_sorted property test to call the sort function before checking if the result is sorted.

Without calling the function under test, the property only checks whether the random input is already sorted, which doesn’t make sense. This change ensures we’re testing the actual behavior of the sort function.

It follows the same approach used in the keep_length property test, where the function is explicitly called to check that the sort function doesn't change the length of the provided input.

Originally proposed in: #324