Changing `Assert.Throws` to `AssertEx.Throws` here simply because I was in the file and I'm having a change of heart about preferring `Assert` over `AssertEx`.
Used to prefer `Assert.Throws` over `AssertEx.Throws` simply on the merit of it being a standard `xUnit` assertion.
But now `AssertEx.Throws` for these makes more sense to me as it means we would not have to rewrite the test if the argument validation would ever happen to be handled after an internal await. Granted that seems like a silly thing to ever do, but at least with `AssertEx` it's one less reason to ever have to rewrite the test.