A pregnant pathologist was found beaten to death and possibly raped in her laboratory office at historic Bellevue Hospital in New York, and officials declared the investigation the city’s No. 1 police priority. Police Commissioner Benjamin Ward assigned a team of 50 detectives to the case, and Mayor Edward I. Koch announced a $30,000 reward for the murderer. “We don’t want doctors looking over their shoulders in fear,” Koch said at a hospital news conference. Security guards, alerted by the woman’s husband, Eric Johnson, found the body of Dr. Kathryn Hinnant, 34, beaten to death in a secluded office where she was preparing a slide presentation for a lecture today, police said.