Is a 12-gauge 50-foot extension cord safe for a 12-amp window AC unit?

Yes, you’re good. 12-gauge wire is rated for 20 amps, so 12 amps is well within spec even at 50 feet, with negligible voltage drop under 2%. Just make sure the cord is rated for outdoor use if it goes through a window, and don’t daisy-chain or coil it tightly while running.