fix(laravel): skip relation metadata for abstract Eloquent models

[cay89]

Q	A
Branch?	4.3
Tickets	Closes #7911
License	MIT
Doc PR	n/a

RelationMetadataLoader crashes with Cannot instantiate abstract class when an API resource model extends an abstract base model.

The Symfony serializer's ClassMetadataFactory walks the full class hierarchy when building metadata. For a chain like ItemTranslation extends AbstractModel extends Model, the loader is also invoked for AbstractModel. Since AbstractModel is still a subclass of Model, the existing is_a(..., Model::class, true) guard passes, and $refl->newInstanceWithoutConstructor() then throws because the class is abstract.

This PR adds an early return false when the reflected class is abstract — there is nothing meaningful to load on an abstract model anyway, and the recursion continues normally for the concrete subclass.

Changes

• src/Laravel/Eloquent/Serializer/Mapping/Loader/RelationMetadataLoader.php: guard with ReflectionClass::isAbstract() before instantiation.
• New regression test: src/Laravel/Tests/Eloquent/Serializer/Mapping/Loader/RelationMetadataLoaderTest.php.
• New workbench fixture: src/Laravel/workbench/app/Models/AbstractModel.php (generic abstract Eloquent model, reusable by future tests).

Backwards compatibility

No BC break: previously, calling the loader with an abstract model class always threw Error: Cannot instantiate abstract class. There is no existing code path that relied on this behavior.