FIX #36312 Prevent circular reference in category parent assignment

[Steph501]

Summary

• Add isDescendant() method to Categorie class to check if a category is a descendant of the current one
• Add validation in create() and update() methods to prevent setting a descendant as parent
• Add translation keys for error message (EN/FR)

Fixes #36312

Test plan

• Normal category creation/update works without errors
• API call trying to set a descendant as parent returns error 500 with message "ErrorCategoryParentIsDescendant"
• Interface already filters the selector, this fix adds server-side protection for API and imports

[Steph501]

The PHPStan failure is not related to this PR.

The errors are:

• Property CommonObject::$shipping_method_id (int) in isset() is not nullable.
• Property Propal::$availability_id (int) in isset() is not nullable.

These are pre-existing issues in CommonObject and Propal classes, not in the category code modified by this PR.