Validate @phpstan-sealed types are subtypes of the annotated type

[klimick]

Validate that every type listed in @phpstan-sealed is a subtype of the annotated class or interface.
Invalid entries are reported with the sealed.notSubtype error identifier.

Add regression coverage for valid and invalid enum, interface, abstract-class, and final-class sealed subtype declarations, including the reflexive case.

Closes #15204

[VincentLanglet]

See phpstan/phpstan#15204 (comment)

[klimick]

You're right, my original constraint was too strict. I agree that a non-final class can represent an open branch of the hierarchy, even if it is not itself a subtype of the annotated type.

I relaxed the validation accordingly: PHPStan now reports an error only when a listed final class or enum is not a subtype of the annotated type. I also added regression tests covering both the valid non-final branch and the invalid final-class case.

I also agree that this should be introduced in bleeding edge, and added the check there.