BARI, Italy — President Donald Trump may be on the outs with Prime Minister Giorgia Meloni, his onetime pal. But whether in the Bay of Naples, the isles of Venice or the southern port town of Bari, Tilman Fertitta, the Texas billionaire who serves as the U.S. ambassador to Rome, is singing a siren song to Italy’s political classes aboard his 383-foot mega yacht, the Boardwalk.