WellPoint Inc.’s $4.9 billion deal for Amerigroup is a bet that health insurers can profit from Medicaid coverage for the poor. WellPoint, the second-largest US health insurer, said it would buy the rival carrier for $92 a share in cash, a 43 percent premium to Amerigroup’s closing price July 6. Amerigroup runs Medicaid coverage in 13 states, including Texas, Florida, and New York.