A rare public accusation of sexual assault aimed at a retired senior Chinese official has rocketed around China’s internet and turned a spotlight on perceived abuses by powerful political figures. Peng Shuai, a professional tennis star, accused Zhang Gaoli, who served as vice premier from 2013 to 2018, of sexual assault in a post Tuesday night on her verified account on China’s Twitter-like Weibo platform, where she has more than half a million followers. In the post, which disappeared after roughly 20 minutes, Ms. Peng, 35 years old, described an on-and-off relationship with Mr. Zhang lasting for some years that…