ERT Medic KNPCs get deadlocked trying to acquire a weapon

Due to a missing check, KNPCs don't check whether a gun they find during /datum/ai_goal/human/acquire_weapon/action is actually a weapon, which leads to them trying to retrieve a med beam gun. They also don't seem to be able to drop the medkit in their active hand or switch hands.

## Reproduction:
Spawn faction_selection=nanotrassen boarders until you get an ert medic.

You will see the "rummages in their backpack" message being spammed. If you investigate with the debugger you will notice they hold a medkit, notice it's not a weapon, search for a gun on their person, find the medigun (in their hand, at least when it's run once before or maybe when they spawn with it already in their hand), try to put it into their active hand, fail because it's holding a medkit, repeat.