[FIX] event: do not suggest public user email in registration chatter

Chatter suggests to include recipients on several objects, notably on
registrations. When a registration is done through a public interface
like website in website_event associated partner is the public user.
In that case proposing to mail the public user does not make sense.
It should propose the real email stored on the registration record.
To fix that we filter the partner if it is linked to the pubic groups.

The fix includes a sudo + context switch in a loop. As this method is
called only on a recordset of one element this has no impact on real use
case. Purpose is to keep the diff minimal.

Closes #23187 .

addons/event/models/event.py

@@ -390,9 +390,14 @@ class EventRegistration(models.Model):
     @api.multi
     def message_get_suggested_recipients(self):
         recipients = super(EventRegistration, self).message_get_suggested_recipients()
+        public_users = self.env['res.users'].sudo()
+        public_groups = self.env.ref("base.group_public", raise_if_not_found=False)
+        if public_groups:
+            public_users = public_groups.sudo().with_context(active_test=False).mapped("users")
         try:
             for attendee in self:
-                if attendee.partner_id:
+                is_public = attendee.sudo().with_context(active_test=False).partner_id.user_ids in public_users if public_users else False
+                if attendee.partner_id and not is_public:
                     attendee._message_add_suggested_recipient(recipients, partner=attendee.partner_id, reason=_('Customer'))
                 elif attendee.email:
                     attendee._message_add_suggested_recipient(recipients, email=attendee.email, reason=_('Customer Email'))