Generic website builder tests were moved into the shared HTML builder area and updated to use common test tools. This helps reduce duplication, keeps testing better organized, and speeds up related test runs without changing customer-facing behavior.

The system now relies on a single built-in safeguard to prevent circular parent-child record links, removing duplicate checks in affected apps. Users may see clearer error messages when they accidentally create a loop, while behavior remains otherwise unchanged.

The ORM's `_parent_store` mechanism already prevents recursive loops by raising a `ValidationError` early on.

This commit removes now-redundant (since
https://github.com/odoo/odoo/pull/35659) Python constraints across various models that were performing the same check.

Additionally, the generic ORM error message for recursion has been made more user-friendly to improve the user experience.