As children across the country drop letters to Santa into mailboxes this week, postal workers are gearing up for one of their busiest times of the year — and one Bay Area letter carrier is embracing the rush with a smile that residents say rivals Santa’s. On a cool December morning, USPS letter carrier Chyanne Thomas moved along her Saratoga route, delivering mail with a cheerful energy and a contagious smile that has endeared her to the community.