Sanaa to Karachi is like 2500 km. Yemen have already demonstrated from its attacks on Israel that they can do long range attacks as well.
Karachi is already in range of their attack drones while only 500 km short from their current missile inventory. But that may change any day if they announced a new longer range missile.
Pakistan have no naval assets or infrastructure to detect these launches timely and respond to them. I think Pakistan would be a fool to become part of this war, specially considering
1. It is not an existential war for KSA.
2. KSA is the one who started it. But it capable of managing it by its own.
3. Pakistan will get severe response from Yemen in term of active attacks.
@Be dazzled