Federal investigators have found that Medicare officials rarely enforce rules for private insurance plans intended to make sure beneficiaries will be able to see a doctor when they need care. It’s a problem many Connecticut seniors know too well. In 2013, UnitedHealthcare, the nation’s largest health insurance company, dropped hundreds of health care providers from its Connecticut Medicare Advantage plan, including 1,200 doctors at the Yale Medical Group and Yale-New Haven Hospital. Medicare Advantage beneficiaries scrambled to find new insurance or new doctors while the Fairfield and Hartford counties medical associations went to court to try to stop the terminations. The report by the Government Accountability Office, the investigative arm of Congress, said that Medicare did not check provider networks to ensure that doctors were available to beneficiaries and cited Connecticut as a “case study” in what can go wrong.