SHANGHAI (Reuters) -Shanghai on Friday announced a record 21,000 new cases and a third consecutive day of COVID testing as a lockdown of its 26 million people showed no sign of easing and other Chinese cities tightened curbs – even in places with no recent infections. Beijing authorities intervened in Shanghai after its failure to isolate COVID by locking the city down in stages, and insists that the country stick to its zero-tolerance policy to prevent its medical system from breaking down. Authorities across China, which have mostly managed to keep COVID at bay for the last two years, are st…