Fix the CI phpstan test

I opted to make the constructor final rather than the entire class, because it may be that someone builds on top of this module and wants to extend the validator class.

I'm happy with that. A pit puzzled by where this static is, I can't see one :)

Ok, great. I think the point of this rule is "lock down the class somehow so that people don't extend it haphazardly, making debugging harder." But to do that they have confusingly named rules... =)

Automatically closed - issue fixed for 2 weeks with no activity.