feat: Enhance UpdateCategory to handle ParentID logic

[gustaavik]

This pull request introduces changes to the category update functionality, focusing on handling the ParentID field, ensuring proper database updates, and adding comprehensive tests. The most important changes include modifying the logic to handle ParentID as nil when set to 0, improving database update behavior for ParentID, and adding tests to validate these behaviors.

Changes to ParentID handling:

• internal/application/usecase/category_usecase.go: Updated UpdateCategory logic to interpret ParentID as nil when set to 0, ensuring proper removal of parent associations. Added validation to prevent circular references and refetch updated categories to maintain data integrity. [1] [2] [3] [4]

Database update improvements:

• internal/infrastructure/repository/gorm/category_repository.go: Modified the Update method to use explicit field updates (name, description, parent_id) to ensure ParentID is correctly updated when set to nil.
• internal/infrastructure/repository/gorm/category_repository.go: Changed the Delete method to use Unscoped deletes, ensuring soft-deleted categories are removed completely.

Test additions:

• internal/application/usecase/category_usecase_test.go: Added a test case TestCategoryUseCase_UpdateCategory_ParentIDZero to verify that setting ParentID to 0 removes the parent association.
• internal/infrastructure/repository/gorm/category_repository_test.go: Added multiple test cases to validate ParentID updates, including transitions from nil to valid IDs, valid IDs to nil, and switching between valid IDs.