[FIX] website_mass_mailing: Use the correct model to unsubscribe

Purpose
=======

When a contact in a mailing list receives a mass_mailing with an
unsubscription link, then he is automatically unsubscribed once the
unsubscription link in clicked instead of opening the
unsubscription form.

Specification
=============

Now the user can decide wether to unsubscribe or not.

Closes #24969

addons/website_mass_mailing/controllers/main.py

@@ -11,7 +11,7 @@ class MassMailController(MassMailController):
     def mailing(self, mailing_id, email=None, res_id=None, **post):
         mailing = request.env['mail.mass_mailing'].sudo().browse(mailing_id)
         if mailing.exists():
-            if mailing.mailing_model_name == 'mail.mass_mailing.contact':
+            if mailing.mailing_model_real == 'mail.mass_mailing.contact':
                 contacts = request.env['mail.mass_mailing.contact'].sudo().search([('email', '=', email)])
                 return request.render('website_mass_mailing.page_unsubscribe', {
                     'contacts': contacts,