HANOI, Vietnam — At Pham Thanh Hong’s dragon fruit orchard in Vietnam, most of the lights are turned off. All is silent except for the periodic thud of the ripe pink fruit falling to the ground. Mr. Pham, 46, is not bothering to harvest them. The farmer watched dragon fruit prices plummet by 25 percent in the last week of December to near zero, pushed down by what several officials in Vietnam say is China’s “zero-Covid” policy. “I’m too disheartened to use my strength to pick them up, then throw…