LONDON— Keir Starmer’s embattled government on Monday published a fresh tranche of files relating to Peter Mandelson’s short-lived and controversial time as Britain’s envoy to Washington. More than 1,000 pages of internal government documents and messages shed light on the Labour grandee’s security vetting for the diplomatic job, his interactions with — and withering verdicts on — Starmer’s top team, and the run-up to and aftermath of his dismissal from his post. Mandelson was sacked as Britain’