NEW DELHI — Allegations of corruption in the purchase of 12 Italian helicopters are threatening India's desperately needed multibillion-dollar modernization of its defense forces. The chief executive of the Italian defense and aerospace company Finmeccanica, Giuseppe Orsi, was arrested in Milan this week on charges of bribing Indian officials to secure a $750 million deal in 2010 to sell the AW-101 medium-lift helicopters.