Tencent Holdings is opening WeChat to smartphone artificial-intelligence agents, as the Chinese tech giant fights to keep the app’s 1.4 billion users locked into its ecosystem in the AI era. The new feature would allow a phone’s built-in voice assistant to send messages or start voice and video calls inside WeChat, according to a report on Thursday by Chinese financial news outlet Yicai. WeChat had been working with the country’s biggest phone makers – including Huawei Technologies, Honor,...