China’s export growth unexpectedly slowed in September, curbing a trade rebound that has been a bright spot for a weakening economy that policymakers are relying on manufacturing to power. Exports climbed just 2.4% in dollar terms from a year earlier to almost $304 billion, the lowest level since May, the customs administration said Monday. Shipments to key markets including Japan, South Korea and Taiwan all fell, while exports to the European Union and the US marked their slowest rise in at least four months as politicians ramped up tariffs.