My first thought when I read this post was: Is his goal to test the code, or validate the features?
The first problem is he's providing the code, and asking for tests. If his code has a bug, the tests will enshrine those bugs. It's like me writing some code, and then giving it to a junior colleague, not providing any context, and saying "Hey, write some tests for this."
This is backwards. I'm not a TDD guy, but you should think of your test cases independent of your code.
Adding tests that capture the current state of things, so that when that bug is uncovered tests can easily be updated to the correct functionality to prove the bug prior to fixing it is a much better place to be than the status quo.
The horse may have bolted from the barn, but we can at least close the farm gate in the hopes of recapturing it eventually.
If your goal is to break your code, try fuzzing. For some reason, it seems that the only people who do it are in the field of cybersecurity. Fuzzing can do more than find vulnerabilities.
You can provide the context to an AI model though, you can share the source with it.