CHICAGO — A longtime Chicago political operative pleaded guilty Friday to a scheme to bribe then-state Sen. Martin Sandoval on behalf of a suburban construction company that needed state approval for a development in East Dundee. William Helm, the onetime deputy commissioner of the Chicago Department of Aviation and a former state transportation official, also admitted in a plea agreement with prosecutors that he and others helped arrange $40,000 in bribes to other, unnamed officials. He also pleaded guilty to a tax count showing he vastly underreported his income over a five-year period, cost…