Fix `Model->isRelation` Method to Accurately Identify Relationships

[o-kima]

This PR addresses an issue in the isRelation method of the HasAttributes trait, where the method would incorrectly return true for any key that exists as a method on the model. This caused issues when determining whether a given key represents a relation.

Previously, isRelation could mistakenly identify non-relationship methods as relationships, leading to unexpected behavior in Eloquent models. This fix ensures that only methods returning a Relation subclass are treated as relations, improving the reliability and accuracy of relationship handling in Eloquent models.

[rodrigopedra]

This will only work if a developer defines a return type for all of their relation methods. Which isn't enforced neither by Laravel, nor by PHP.

Therefore, it is a breaking change, a huge one, and might be the reason that so many tests are failing.

If there is a defined return type, I agree we could have this additional check. But for methods that don't have a defined return type, the old behavior should be kept.

[crynobone]

Setting this to draft until tests are passing (and without introducing any breaking changes)

[MahdiSaremi]

These changes will break the production programs! With this change, the return type must be defined in all relations.