More than 100 legal immigrants who were supposed to start receiving care Dec. 1 under a new Massachusetts health insurance plan will have to wait because the contractor hired to run the program has been unable to reach agreement with providers in Western Massachusetts. The immigrants, 123 in Pittsfield and 16 in Adams, are among roughly 5,000 in western and central Massachusetts who became eligible for coverage under the CeltiCare Health Plan of Massachusetts. But CeltiCare has been unable to negotiate contracts with hospitals in those two cities, said a state spokesman.