After years shunning Iran, Western businesses are bursting through the country’s doors—but U.S. companies are noticeably absent. Dozens of development projects and deals have been hammered out since Iran’s nuclear accord with world powers in 2015 lifted a range of sanctions. Among them, France’s Peugeot and Renault SA are building cars. The U.K.’s Vodafone Group PLC is teaming up with an Iranian firm to build up network infrastructure. Major oil companies including Royal Dutch Shell PLC have signed provisional agreements…