

No, it is not an act of war. Article 51 of the UN Charter explicitly grant right to self defence. Inviting other nations to assist in that defence is a right granted by the UN Charter. Iran can obviously consider Ukranian personal as hostile, that is their prerogative, but that does not mean that shooting down drone in self defence is an act of war.
Iran is the aggressor against the gulf state. No gulf state has made aggressive action against Iran, and there was no military action from gulf states except self defence. All gulf states have no interest in getting involved in the conflict and asked for deescalation at the UN. In a joint declaration from the middle east council on global affairs:
When Iran decided to attach civilian targets totally unrelated with the conflict then it became aggressor. Iran can be the aggressor and the target of war at the same time.