SAN JOSE — One-by-one they rappelled off a landmark Adobe building downtown Saturday, carefully traversing its 16 floors as onlookers cheered and snapped photos from down below. Some, looking to add a bit of flair to the tricky challenge, did so in full costume (there were a few Spider-Man sightings). It’s not something you see a local politician do everyday, let alone a tech executive. There was good reason for the early-morning spectacle: The first-ever Downtown Drop Down “rappel-a-thon” benefited Downtown Streets Team, a San Jose-based nonprofit that runs a program aimed at giving homeless volunteers work experience. More than 90 people, including four local mayors, rappelled the 16 floors — 236 feet — to support the nonprofit, which was trying to raise $275,000 for programs supporting homeless people across eight Bay Area locations.