NEW YORK — Negotiations to keep 10,000 New York City nurses from walking off the job headed into a final weekend as some major hospitals were already preparing Friday for a potential strike by sending ambulances elsewhere and transferring some patients, including vulnerable newborns. “New York City hospitals have violated our trust through years of understaffing, and that understaffing has only gotten worse since the start of the COVID-19 pandemic,” nurses’ union President Nancy Hagans said at a news briefing Friday. Nurses at a Massachusetts hospital went on strike for nearly 10 months ending last January, marking the longest nursing walkout in state history.