What began with a plan to replace an aging piece of medical equipment has turned into a dispute over the delivery of cancer care along Connecticut's shoreline. And at a time when policymakers have expressed worries about preserving competition in the state's fast-consolidating health care market, one side has suggested the case highlights questions about competition — and the way state regulation can limit it. As officials at Middlesex Hospital tell it, they have a unique opportunity: One of their two linear accelerators, used in radiation treatment for cancer, is at the end of its useful life.