BOSTON — The Boston City Council is planning to hold a public hearing on one of the largest hospital mergers ever proposed in Massachusetts.The 13-hospital deal would result in the merger of Beth Israel Deaconess Medical Center hospitals, The Lahey Health system, along with New England Baptist Hospital in Boston, Mount Auburn Hospital in Cambridge, and Anna Jaques Hospital in Newburyport.The hospitals hope to create a health network that can compete with Partners HealthCare, the parent company of Massachusetts General and Brigham and Women's hospitals.