NEWYou can now listen to Fox News articles! Manhattan prosecutors are signaling they could fight a federal plea deal for Luigi Mangione if it threatens to derail his upcoming New York murder trial in the killing of UnitedHealthcare CEO Brian Thompson.In a newly unsealed letter reviewed by Fox News Digital, the Manhattan District Attorney’s Office said it is aware of reports that Mangione could seek a guilty plea in his separate federal case before the state trial begins.Prosecutors warned that if such a deal would interfere with New York’s prosecution, they could ask the federal judge to take that into…