China plans to cooperate with Saudi Arabia and other Gulf countries in the fields of nuclear energy, nuclear security and space exploration, President Xi Jinping said on Friday, showcasing his nation’s strengthening ties with a region that was once firmly in the U.S. sphere of influence. Mr. Xi was speaking in the Saudi capital, Riyadh, at a summit with rulers and officials from the six Gulf Cooperation Council countries — Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates — during a three-day visit to Saudi Arabia. Later on Friday, he held his third and final summit of…