HHH-20827 - Fix lazy loading for abstract @ConcreteProxy entities

[yashagraawal]

Summary:
Fixes lazy loading for @manytoone associations targeting abstract @ConcreteProxy entities when bytecode enhancement is disabled.

Problem:

• Lazy @manytoone references to abstract entities annotated with @ConcreteProxy were being eagerly loaded
• Hibernate was not creating proxy factories for abstract @ConcreteProxy entities, assuming proxies weren't needed for abstract classes
• On JDK 25, attempts to create proxies for sealed classes caused IncompatibleClassChangeError

Solution:

1. Create ProxyFactory for abstract @ConcreteProxy entities - Enable proxy creation when the entity is abstract, uses @ConcreteProxy, is not sealed, and lazy loading is enabled
2. Exclude sealed classes from proxy creation - Add sealed class checks to prevent ByteBuddy errors on JDK 25
3. Select discriminator for @ConcreteProxy entities - Ensure discriminator columns are selected for @ConcreteProxy entities with subclasses so Hibernate can resolve the concrete entity type

Testing:

• All 23 ConcreteProxy tests pass
• ConcreteProxyWithSealedClassesTest passes (3/3) - JDK 25 compatibility
• New test: testAbstractSingleTable() validates abstract @ConcreteProxy lazy loading
• Tested on JDK 17, 21, and 25

Files Changed:

• EntityRepresentationStrategyPojoStandard.java - Proxy factory creation for abstract @ConcreteProxy + sealed class exclusion
• EntityFetchSelectImpl.java - Discriminator selection for @ConcreteProxy with subclasses
• ConcreteProxyTest.java - Test coverage for abstract @ConcreteProxy entities

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Please make sure that the following tasks are completed:
Tasks specific to HHH-20827 (Bug):

• Add test reproducing the bug
• Add entries as relevant to migration-guide.adoc OR check there are no breaking changes

https://hibernate.atlassian.net/browse/HHH-20827

[github-actions]

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[yashagraawal]

Hi @beikov, I see a GH action (GraalVM 25 - autonomous-transaction-processing-serverless-19c) failing with 504, could you please confirm and re-trigger it ? Thanks!

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
80.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud