BRUSSELS — EU countries are rethinking how they would respond if one of them is attacked — but those closest to Russia fear it could blur NATO’s role. Kaja Kallas, the bloc's top diplomat, will brief EU leaders on Thursday on Article 42.7 of the EU Treaty, which obliges member states to provide aid and assistance “by all the means in their power” if another EU country is attacked. The problem: no one is quite sure how it would work. Officials are now gaming out how to use the clause in a crisis,