Added Error Code for HRIS Rejections when Pushing Candidate Data

candidate.pushToHris now returns a specific candidate_hris_push_failed error code instead of a generic internal_server_error when the third-party HRIS does not accept the push (e.g. a data or validation discrepancy reported by the downstream system, such as Workday). The message field includes detail about the failure (typically the error reported by the HRIS) so it is actionable. Because the endpoint pushes with re-push enabled, if any of the data was already applied in the external system, verify there before retrying to avoid a duplicate.