By Nate Raymond (Reuters) – The U.S. Supreme Court on Tuesday wrestled with the circumstances under which doctors can be convicted of operating as drug dealers under the cover of their medical practices to illegally distribute opioid painkillers and other dangerous narcotics. The justices heard arguments in an appeal by two doctors, Xiulu Ruan and Shakeel Kahn, of lower court rulings upholding their convictions on narcotics violations and related crimes stemming from what prosecutors called the misuse of medical licenses to engage in drug trafficking. Lawyers for Ruan, who practiced in Alabama…