By Moira Warburton and Jan Wolfe WASHINGTON (Reuters) -Former U.S. President Donald Trump on Saturday said he is endorsing celebrity surgeon Dr. Mehmet Oz, who is running as a Republican in a closely watched U.S. Senate contest in Pennsylvania. “Dr. Oz is smart, tough, and will never let you down, therefore, he has my Complete and Total Endorsement,” Trump said in a statement. The Senate race in Pennsylvania could determine control of the Senate and the fate of Democratic President Joe Biden’s agenda. A crowded field of candidates are vying to replace the retiring Republican Senator Pat Toomey…