Federal officials Sunday night turned on a portion of HealthCare.gov to allow consumers to explore the benefits and prices of health plans that will be sold for 2015 through the federal health insurance exchange. The window-shopping period is beginning several days before the Nov. 15 official start of the sign-up period for people in the three dozen states that rely on the federal exchange to renew their coverage or buy insurance for the first time. Andy Slavitt, principal deputy administrator of the Centers for Medicare and Medicaid Services, the agency overseeing the marketplace, said technical staff working on HealthCare.gov rebuilt this portion of the Web site to make it easier to use.