A Texas-based hospital chain that is buying four Connecticut hospitals said in a surprise announcement Tuesday it will add a fifth to its roster: Saint Mary's in Waterbury. Connecticut's legislature this year passed a bill allowing for-profit hospitals to employ doctors, removing a hurdle that jeopardized Tenet Healthcare Corp.'s plans to buy four nonprofit hospitals in Connecticut: Waterbury, Bristol, Manchester Memorial and Rockville General. Tenet Healthcare said it had been in talks privately with executives at Saint Mary's for some time, but the hospital's board had to approve the acquisition, which it did last week.