TIJUANA, Mexico (Reuters) - A teenager was injured when U.S. border patrol agents fired tear gas into Mexico over the weekend, despite a Mexican government complaint that U.S. agents were using excessive force. Fifteen-year-old Cristian Saldana was hit in the nose by a tear gas canister on Saturday after he threw stones at border patrol agents trying to stop people from illegally crossing into the United States, witnesses said. The teenager required stitches at a local hospital. Mexico's government criticized the U.S. border patrol for using excessive force after border patrol agents fired tear gas in another incident on Friday....