The Houthis’ rapid advance toward one of the world’s most important shipping chokepoints is putting fresh scrutiny on the sprawling financial network that has helped transform the Iran-backed group from a Yemeni insurgency into a heavily armed regional power capable of threatening global trade. The group has seized new territory along Yemen’s Red Sea coast, including the strategic port city of Mocha, and expanded toward the Bab el-Mandeb Strait.…