There's no global definition of what does and does not constitute an act of war and there's no international body that forces other countries to declare war on other countries. Something is considered an act of war if a country claims that it is. That's it. Giving equipment to Ukraine to fight Russia is not an act of war because Russia has (at least not yet) said they consider to be. That's all there is to it. Russia is pefectly free to say "we consider this an act of war, and so therefore we are not at war with all of NATO," but they don't want to be at war with NATO, so they don't.