WASHINGTON — President Joe Biden tested positive for COVID-19 on Thursday and is experiencing “very mild symptoms,” the White House said, as new variants of the highly contagious virus challenge the nation’s efforts to get back to normal after two and a half years of pandemic disruptions.White House Press Secretary Karine Jean-Pierre said Biden has begun taking Paxlovid, an antiviral drug designed to reduce the severity of the disease. He was isolating at the White House and “continuing to …