Medicaid coverage provider Centene will pay about $6.3 billion to buy fellow insurer Health Net, as managed-care companies look to bulk up while adjusting to the federal health care overhaul. The deal announced Thursday morning gives St. Lous-based Centene a chance to grow in two hot areas for health insurers, the state- and federally funded Medicaid program for the poor and people with disabilities, and the federally funded Medicare Advantage program, which has seen its overall enrollment triple over the past decade.