feat: 🎸 Refactor modeling

Remove OIDC as default value for fragment field filter. Applied default
value OIDC to type field. Accomodate tests for the changes. Add a
readOnly serializer test. Delete from fragment serializer not
serializing readOnly, because that is a case it will never reproduce on
such model.

addons/api/addon/generated/models/managed-groups.js

@@ -36,6 +36,7 @@ export default class GeneratedManagedGroupModel extends BaseModel {
 
   @attr('string', {
     description: 'The type of this ManagedGroup.',
+    defaultValue: () => 'OIDC',
   })
   type;