A new contract struck between Partners HealthCare System Inc. and Tufts Health Plan will help control health care costs in Massachusetts by limiting payment increases to Partners' hospitals and doctors to the rate of inflation, both sides said Wednesday. Partners, the state's largest hospital and physicians organization, tore up the last two years of its existing contract with the health insurer, replacing the pact with a new four-year agreement that, while still giving Partners more money each year, will lower Tufts' reimbursements to the medical care provider by about $105 million from what they would have been under the former rate structure.