The man who served as Matthew Perry’s personal assistant — and who prosecutors said injected the actor with the ketamine dose that proved fatal — was sentenced Wednesday to 41 months in federal prison, marking the end of the criminal case against five people who admitted roles in the Friends star’s death. U.S. District Judge Sherilyn Garnett handed down the sentence to Kenneth Iwamasa, who discovered Perry unresponsive, floating face down in a hot tub at the actor’s Los Angeles home in October 2023. Prosecutors said Iwamasa…