RIYADH, Saudi Arabia — Saudi Arabia and China signed a strategic partnership agreement on Thursday during a visit by the Chinese leader Xi Jinping to the kingdom, underlining the growing ties between Beijing and a longstanding American ally. Mr. Xi held talks with Crown Prince Mohammed bin Salman, the prime minister and de facto ruler of Saudi Arabia, on Thursday in the first of a series of summits planned for the Chinese president’s three-day visit. After his bilateral meetings with Saudi officials, Mr. Xi is expected to attend twin summits…