UBS on Wednesday became the first big global bank in more than two decades to have a subsidiary plead guilty to fraud. UBS, the Swiss bank, scrambled until the last minute to avoid that fate. A week ago, in a bid for leniency over interest-rate manipulation, the bank’s chairman traveled to Washington to plead his case to the Justice Department, according to people briefed on the matter. Knowing the long odds, the chairman, Axel Weber, asked the criminal division for a lighter punishment.