I have removed the "Notes" from Kata description as this doesn't apply anymore beacause I have removed the input validation. Read the comment from sakana378.

Input validation and its test cases are removed. I also removed the "Notes" from Kata description as this doesn't apply anymore.

I totally agree with you guys! I will remove the input validation and the tests that cover this feature.
Instead, for the sake of good programming habit, I will throw an exception in Circle() in the case when the input is invalid.

Now I see, you are referring to the "Notes" section of the Kata description. Right. I will add a necessary info to this section to be clear.

I suppose you mean the test suite assert messages? Agreed. I also think that they are rather... dull and not informative enough.

It looks good to me. Thank you!

I also thought about that while I was writing the test cases. But for me, it didn't make sense for 0-radius circles to collide even if they are located at the same coordinates. My point of view is that 0-radius circles effectively don't exist in a plane. Hence they can not collide.

Above is true even if both circles have a radius set to 0 but share the same coordinates. I hope you that you can see my point.

Thank you for sugestions. Both are valid and will be corrected. I am currently experiencing issues (unable to republish the Kata) with the Codewars platform. I'll try later during the day.