✅(tests) fix tests after adding slugs to domains

- slug readonly on admin
- fix test to expect slug in payload, when retrieving a domain

src/backend/mailbox_manager/models.py

@@ -28,10 +28,14 @@ class MailDomain(BaseModel):
         return self.name
 
     def save(self, *args, **kwargs):
-        if not self.slug:
-            self.slug = slugify(self.name)
+        """Override save function to compute the slug."""
+        self.slug = self.get_slug()
         return super().save(*args, **kwargs)
 
+    def get_slug(self):
+        """Compute slug value from name."""
+        return slugify(self.name)
+
     def get_abilities(self, user):
         """
         Compute and return abilities for a given user on the domain.