Keep in mind that there is a lot of propaganda right now about how the assassination was carried out. The story about the bomb planted in the room is an attempt to say that the act was a targeted intelligence action which would limit the scope and options for Iran to respond.
If it was a rocket, Iran has the right to respond under Section 51 of the UN Charter:
"Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security."
In fact Iran has written to the UN to inform them of their plan to respond accordingly under Section 51. https://x.com/Divsallar/status/1818709341848891406
Remember Hezbollah has basically gone dark and is refusing to speak to Western mediators, yesterday Nasrallah openly stated that there will a be a response stronger and larger than the current stance in South Lebanon.
Yemen is itching to respond to the attack on their harbour.
Iraq is ready too.
Don't forget that the Iranian response in April was carried out after mediation through third parties with the US to limit the scope and Iran provided notice. That attack also allowed Iran to "see" the type of Air Defence that Israel has.
Basically the whole "resistance axis" have changed their stance from providing a support front for Gaza to responding to these attacks. Additionally from Axios:
"The Biden administration is concerned it may be more difficult to mobilize the same international and regional coalition of countries that defended Israel from the previous Iranian attack because Haniyeh's assassination is in the context of the Israel-Hamas war, which has drawn sharp anti-Israel sentiments across the region.
https://www.axios.com/2024/08/01/us-iran-israel-attack-preparation