WASHINGTON — One Supreme Court justice explained her absence from a case. One justice didn’t.The difference shows how difficult forging consensus over even small steps on ethics can be at the Supreme Court, which is facing new calls to adopt an ethics code following revelations about undisclosed gifts from a Republican megadonor to Justice Clarence Thomas.Last week, Chief Justice John Roberts acknowledged the court needs to do more to reassure a skeptical public that the justices take their ethical obligations …