By Guy Faulconbridge and Nandita Bose MOSCOW/WARSAW (Reuters) -China pledged to deepen cooperation with Russia on Wednesday while U.S. President Joe Biden discussed security with leaders of NATO's eastern flank, highlighting geopolitical tensions as the anniversary of Russia's invasion of Ukraine approaches. Within Ukraine, schools took their classes online for the rest of the week for fear of an upsurge of Russian missile attacks a year on from Moscow's Feb. 24 all-out assault, which failed to topple the government and has long been bogged down. Making the highest level visit to Russia by a C...