✅(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/api/serializers.py

@@ -26,6 +26,12 @@ class MailDomainSerializer(serializers.ModelSerializer):
             "created_at",
             "updated_at",
         ]
+        read_only_fields = [
+            "id",
+            "slug",
+            "created_at",
+            "updated_at",
+        ]
 
 
 class MailDomainAccessSerializer(serializers.ModelSerializer):