KYIV, Ukraine — Russia seized on the mass surrender of Ukrainian troops at a Mariupol steel plant as a propaganda gift on Wednesday, moving to falsely label them as terrorists and create a parallel narrative to Ukraine’s portrayal of Russian soldiers as heinous war criminals. The mass surrender, which ended the longest battle of the three-month-old war, was depicted by the Russians as a glorious turning point in a conflict that Western military analysts and rights groups have described as disastrous for the Kremlin and its forces, which have bombed Ukraine indiscriminately and been accused of other atrocities. Images of...