PARIS — Marat Gabidullin's face is lined from years of exposure to the elements, and his hair is thinning. But at 56, he has the trim physique and muscular arms of a man 30 years younger. He wears a chunky ring bearing the image of a skull.The skull is the symbol of the Wagner Group — a private Russian mercenary force believed to be financed by an oligarch with close ties to President Vladimir Putin. The group is fighting alongside the Russian army in Ukraine's eastern Donbas region. And it's widely believed that at least some of the "little green...