HELSINKI, Aug 25 (Reuters) - Social media channels linked to Russia's Wagner Group mercenaries said on Friday a top fighter in one of the group's subunits had been detained in Finland at Ukraine's request. Rusich, a far-right subunit affiliated to Wagner, identified him as Yan Petrovsky, a founding member and leader of the unit who has been under European Union and United States sanctions since last year. It said on the messaging app Telegram that Petrovsky, who has fought in Ukraine since 2014, was initially arrested on July 20 for a visa violation, and could now be extradited to Ukraine....