I fixed these now ( I think ).

I could not find a guide/"kata solution best practices" for this anywhere, and I thought that there might be a smart way to include the "solution" into the random test without duplicating the entire code for it.

Thank you for the feedback, this is stil my first cata translation so i'm grateful for the remarks.

Please remove the validation yes.

Is there any good way to add Fuzz tests on Codewars without copying the solution into the test class?

That's how you always make random tests! What's the problem of sticking it in there, anyway? Just not in the sample tests.

I did not know about the issue with Int in Elm and had real troubles understanding why the C# test cases did not pass. So much for no runtime errors. I thought this could make others learn about the possible limitation. I'll remove it if you want me to.

Is there any good way to add Fuzz tests on Codewars without copying the solution into the test class?

This is my first attempt at porting a Kata, so I appreciate the patience.

We handle that by returning -1 for values of 2^31 or larger.

What's the point in input validation at all?.. Just don't test those input and that's it.
Random tests should be added.