LONDON — Keir Starmer’s team believed he was the perfect candidate to be British Prime Minister when he took office in July 2024. But winning an election is one thing and running the country, it turns out, requires something else.  Stuck in a months-long crisis over his appointment of Peter Mandelson as U.K. ambassador to Washington, Starmer is now stumbling toward what many on his own side fear is the endgame of his premiership, less than two years since leading his Labour Party to a landslide