The open race to lead the nation’s second-largest prosecutor’s office was too close to call Wednesday after a heated primary campaign in the Chicago area.The Democratic matchup for the Cook County state’s attorney primary features Eileen O’Neill Burke, a former appellate judge with a large campaign war chest, versus Clayton Harris III, a professor and attorney who’s held government posts. O’Neill Burke had a narrow lead as late results were still coming in from Chicago.“We are cautiously optimistic, but we have to make …