Original PR description

Steps to Reproduce:
1. Create a database without installing the Website module.
2. Navigate to archived users and delete the "Public User."
3. Attempt to log in to the database from another browser or incognito
    mode.
4. An internal server error occurs because the public user does not
    exist, making the login page inaccessible.

Issue:
Previously, it was possible to delete the public user, leading to an
internal server error due to its absence, which prevented public access
to the login page.

Solution:
- Implemented a restriction to prevent the deletion of the public user,
    similar to portal and default users.
- Added a test case to validate this functionality and ensure the
    public user cannot be deleted.

task-4423568


Forward-Port-Of: odoo/odoo#233328
Forward-Port-Of: odoo/odoo#196918