ELMAU, Germany — The Group of Seven economic powers are set to commit themselves to supporting Ukraine in the long haul, with the U.S. preparing to announce the purchase of an advanced surface-to-air missile system for Kyiv, as leaders meet in the German Alps and confer by video link with Ukrainian President Volodymyr Zelenskyy. The G-7 leaders will begin Monday’s session of their three-day summit with a focus on Ukraine.