BANGKOK — Bobby was a good boy. So was Bravo. Angel was a good girl, and when she sat, furry hindquarters sliding a little on the tile floor, she raised a paw for emphasis, as if to say, It’s this cotton ball that my keen nose has identified, the one that smells like Covid-19. The three Labradors, operating out of a university clinic in Bangkok, are part of a global corps of dogs being trained to sniff out Covid-19 in people. Preliminary studies, conducted in multiple countries, suggest that their detection rate may surpass that of the rapid antigen testing often used in airports and other public places.