SAN FRANCISCO — Google for the first time has uncovered evidence that Russian operatives exploited the company’s platforms in an attempt to interfere in the 2016 election, according to people familiar with the company’s investigation. The Silicon Valley giant has found that tens of thousands of dollars were spent on ads by Russian agents who aimed to spread disinformation across Google’s many products, which include YouTube, as well as advertising associated with Google search, Gmail, and the company’s DoubleClick ad network, the people said, speaking on condition of anonymity to discuss matters that have not been made public.