MIAMI — They had just finished up lunch, and set off to run a humdrum errand: a drive to the travel agency to pick up airline tickets for their annual visit to their beloved homeland, Cuba. Osvaldo Gonzalez and Alberto Arias, friends and business partners, happened to pass under a Miami bridge that Thursday afternoon, the road bustling with fellow drivers also out on the most ordinary and unthreatening of life’s tasks. A teenager was driving her friend to the doctor’s office to pick up some medicine.