[13.x] Fix JsonApiResource relationships resolved through a closure

[markwalet]

PR description is generated by Claude and checked by myself, just wanted to be open about that.

This PR fixes a crash when a JSON:API relationship is resolved through a closure that returns a resource:

public function toRelationships(Request $request): array
{
    return [
        'author' => UserResource::class,
        'comments' => fn () => CommentResource::collection(
            $request->user()->is($this->resource)
                ? $this->comments
                : $this->comments->where('is_public', true),
        ),
    ];
}

This throws the following error:

Illuminate\Http\Resources\JsonApi\RelationResolver::handle(): Return value
must be of type Illuminate\Database\Eloquent\Collection|Illuminate\Database\Eloquent\Model|null,
Illuminate\Http\Resources\JsonApi\AnonymousResourceCollection returned

RelationResolver::handle() only accepted a raw Collection, Model, or null, so a closure returning CommentResource::collection(...) (an AnonymousResourceCollection) blew up on the return type before the value was ever used.

The fix stays inside RelationResolver. A returned resource collection already carries everything we need — the underlying models on its collection, and the resource class on its collects property — so we unwrap it back into the models and adopt that resource class. A single returned resource is unwrapped the same way. This mirrors what the resolver already does internally when you hand it a class-string; the closure form was simply arriving "pre-wrapped" and the resolver didn't know how to open it.

Closures that already return raw models or Eloquent collections keep working unchanged, so this is fully backward compatible — it only adds handling for return types that previously errored.

Tests are included covering a closure that returns a resource collection, a single resource, raw models, and null.

Closes #60121