When he pulled into the parking lot of the Meadow Glen Mall in Medford, State Police Sergeant Brian O'Hare believed he was about to meet the 14-year-old boy he had been courting for months on the Internet, and the two would have sex, authorities said yesterday. But there was no boy, just FBI agents waiting to arrest O'Hare at the end of a six-month undercover sting in which agents posed as flirty teenagers on an AOL chat room dubbed BoysShowerM4, said FBI spokeswoman Gail A. Marcinkiewicz. Well known among his 2,300 peers in the State Police, O'Hare is a 19-year...