The Chinese national flag fluttering with the Lujiazui Financial District in the background.Vcg | Visual China Group | Getty ImagesChina's stock market rebound may be showing signs of strain as renewed U.S.-China trade tensions threaten to derail investor optimism.After months of relative calm, Washington's fresh warnings over Beijing's rare earth export controls and renewed trade tensions have revived fears of another tit-for-tat trade cycle. Chinese shares had recently rallied to a multi-year high on expe ...