Fifteen people, including a lawyer who worked at several major law firms, pleaded not guilty on Monday to federal charges that they participated in a decade-long scheme in which attorneys fed an insider trading ring tips about nearly 30 underway mergers. Nicolo Nourafchan, who had worked at the law firms Sidley Austin, Latham & Watkins and Goodwin Procter, was among the defendants who appeared in federal court in Boston to enter pleas to securities fraud and other charges. Thirty people in