By Jeffrey Dastin and Akash Sriram (Reuters) -Alphabet disappointed Wall Street on Tuesday as holiday-season advertising sales came in below expectations and the company said its spending on items such as servers to power artificial intelligence would jump this year. Alphabet shares fell 6% in after-hours trade. Against a backdrop of mixed U.S. economic signals, Alphabet’s powerhouse units Google and YouTube have faced competition for ad budgets from other online platforms, including Facebook, Instagram, TikTok and Amazon.com. With retail sales a bright spot, the company’s fourth-quarter ad re…