DENVER — The landmark Cadet Chapel that towers over the U.S. Air Force Academy in Colorado is suffering from leaks and corrosion, so the school has drawn up the most ambitious restoration project in the building’s 55-year history. But the repairs have been on hold for months while officials try to settle on a contractor for the long and complicated project. Water has been seeping through seams in the chapel’s aluminum skin for years, said Duane Boyle, the academy’s campus architect.