BRUSSELS — Efforts by European Council President António Costa to open diplomatic backchannels to the Kremlin could undermine the bloc’s support for Ukraine, Estonia’s prime minister has warned in the most staunch condemnation of the move yet. In comments to POLITICO following the first day of a summit of leaders in Brussels, Kristen Michal said Russia continues to reject any realistic path to peace — and insisted any diplomacy must be led by Kyiv. “The European Union cannot assume the role of