fix(metadata): preserve explicit class on ApiResource when propagating defaults

[soyuka]

Summary

When #[ApiResource(class: SomeClass::class)] is set explicitly on a resource, the factory was overwriting the user-provided class with the PHP class name during default propagation. As a result, the explicit class never reached cascaded operations.

The change in OperationDefaultsTrait::getResourceWithDefaults() now falls back to the PHP class name only when no class is already set on the resource.

Maintainer directive (from issue #7187): "you shouldn't really change the class but indeed we should fix this".

Reproduction

#[ApiResource(class: ResourceClassPropagationTarget::class)]
final class ResourceClassPropagation {}

Before the fix, every operation's getClass() returned ResourceClassPropagation::class, ignoring the explicit attribute argument. After the fix, both the resource and every cascaded operation expose ResourceClassPropagationTarget::class.

Test plan

• Added failing test testCreatePropagatesExplicitResourceClass.
• Test passes after the fix.
• Full AttributesResourceMetadataCollectionFactoryTest class still green (12 tests, 31 assertions).
• tests/Functional/OverriddenOperationTest.php still green (10 tests, 20 assertions).
• php-cs-fixer + phpstan clean on changed files.

Fixes #7187