Enforce JPA bootstrap failure for incomplete Hibernate Validator setup

[jhoeller]

Following up on spring-projects/spring-boot#35617 (comment), it seems suboptimal for Hibernate ORM to silently skip validation in case of an incomplete Hibernate Validator setup without a Jakarta EL provider on the classpath. It is too easy to add Hibernate Validator but miss the EL provider, none of which is noticeable through JPA setup.

We could provide custom detection of validation engine presence in DefaultPersistenceUnitManager and set the validation mode to "CALLBACK" in that case, enforcing bootstrap failure for an incomplete setup.