The co-defendant in the federal fraud case against Cook County Circuit Judge Jessica A. O’Brien has pleaded guilty to federal mail fraud charges.Maria Bartko entered her plea at a Friday hearing before U.S. District Judge Thomas A. Durkin at the Dirksen Federal Courthouse. Prosecutors told Durkin they plan to bring Bartko as a government witness against O’Brien during the criminal trial scheduled next week, according to a transcript of the hearing.Both Bartko and O’Brien were indicted by a federal jury …