[FIX] mass_mailing_event: keep default domain

Before this commit, when cliking on "mass mail attendees" on a event form
the default domain on the attendees for determining the mass mailing recipient
was lost in an onchange

After this commit, we retrieve the right domain

OPW 1869308
closes #26081

addons/mass_mailing_event/models/__init__.py

@@ -2,3 +2,4 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 from . import event
+from . import mass_mailing

addons/mass_mailing_event/models/mass_mailing.py

@@ -0,0 +1,12 @@
+from odoo import models, api
+
+
+class MassMailingCampaign(models.Model):
+    _inherit = "mail.mass_mailing"
+
+    @api.onchange('mailing_model_id', 'contact_list_ids')
+    def _onchange_model_and_list(self):
+        result = super(MassMailingCampaign, self)._onchange_model_and_list()
+        if self.mailing_model_name == 'event.registration' and self.mailing_domain == '[]':
+            self.mailing_domain = self.env.context.get('default_mailing_domain', '[]')
+        return result