When a man identifying himself as Chris Chen reached out this winter to an aide on a House committee focused on threats from China, he came armed with a lucrative offer. The staff member, Mr. Chen proposed, could earn $10,000 or more by barely lifting a finger. All he would need to do is agree to phone calls every other week to share information about the committee’s work and U.S. foreign policy about China. Insights into U.S. trade or national-security issues, including the Trump administration’s plans for Venezuela in the…