Add filterCallback to EntityCollectionMappingFilter and EntityMapping

[1on]

Changes

Added support for custom existing-entity resolution in EntityMappingFilter and
EntityCollectionMappingFilter.

Both filters now accept an optional filterCallback. It can be used to define custom
lookup logic, for example resolving an entity by uuid. If the callback is not provided
or does not return an entity, the previous id-based lookup behavior is preserved.

Details

• Added optional filterCallback constructor argument to EntityMappingFilter.
• Added optional filterCallback constructor argument to EntityCollectionMappingFilter
  and pass it to the inner EntityMappingFilter.
• Collection item deletion can now resolve deleted items without an input id:
  • if id is present, the previous behavior is used;
  • otherwise, the filter attempts to resolve the entity through filterCallback and uses
    its getId().
• Covered the existing id lookup behavior with a regression test.
• Moved test helper classes to tests/Utils.

Why

This allows EntityCollectionMappingFilter to support inputs identified by external
identifiers such as uuid, while keeping backward compatibility with the existing
id-based mapping behavior.