Negotiators for Tufts Medical Center and the Massachusetts Nurses Association agreed on an 18-month contract extension early this morning, only hours before 1,100 registered nurses were set to walk off their jobs at the Boston teaching hospital. The tentative deal, reached after 15 hours of bargaining yesterday and overnight, avoided what would have been the first nurses strike at a Boston hospital in 25 years. The nurses association withdrew its strike notice and scheduled a ratification vote for May 19. While it did not resolve the parties' dispute over the union's demand for mandatory staffing levels, the key sticking point in the contentious talks, the contract extension contained language preventing Tufts from assigning more than five patients to a single nurse on the day or evening shift for the life of the pact. The hospital agreed to limit patient assignments to no more than six on the night shift and two in intensive care units during the next year and a half.