By Trevor Hunnicutt, Steve Holland and Jeff Mason WASHINGTON (Reuters) -Joe Biden, the oldest person ever to serve as president of the United States, has tested positive for COVID-19, is experiencing mild symptoms and will continue working but in isolation, the White House said on Thursday. Biden, 79, has a runny nose, fatigue and an occasional dry cough, symptoms which he began to experience late on Wednesday, White House physician Kevin O’Connor said in a note released on Thursday. Biden has begun taking the antiviral treatment Paxlovid, O’Connor said. Fully vaccinated and twice boosted, Bid…