One of the city’s last Gilded Age mansions, which comes with its own unusual history, has sold for exactly $50 million in an all-cash deal, according to listing broker Tristan Harper of Douglas Elliman. The six-story limestone Beaux-Arts mansion at 845 Fifth Ave. — complete with bulletproof windows facing Central Park and Cold War-era spy gear, such as a metal-padded room with a Faraday cage to block electromagnetic fields — was owned by the former Yugoslavian government until war broke out in 1992. It’s also reportedly where Josip Broz Tito, the late head of Yugoslavia, hid out following an assassination attempt against him at the Waldorf Astoria in 1963.  The 30-foot-wide mansion, between…