fix: use instanceof for relation detection

[zigzagdev]

what i have done

This PR replaces the incorrect use of is_subclass_of() with instanceof in the getRelatedModelFromRelation() method.

When the relation method returns an actual instance (f.e. BelongsTo, HasMany, etc.), is_subclass_of() returns false, since it expects a class name string.

This caused false negatives when verifying Eloquent relation types.

why

• is_subclass_of() checks class inheritance on class names, not on instances, leading to incorrect results.
• Using instanceof correctly identifies Relation instances and their subclasses (BelongsTo, HasMany, MorphTo, etc.).
• This change aligns with PHP’s expected type-checking semantics and avoids false negatives.

Tests

throws RelationNotFoundException when the relation method exists but does not return an Eloquent Relation

result