deleteManaged method
Removes a managed user from its parent.
Detaches the link but does NOT delete the underlying user
account; the same external ID can later be re-attached via
createManaged. fromUserId is the parent — required because a
managed user can in principle have multiple parents.
Implementation
@override
Future<ChatResult<void>> deleteManaged(
String userId, {
required String fromUserId,
}) async => const ChatSuccess(null);