AI - Path Finding System?

I'm not aware of anything related to the PFS which will do this for you.

I would recommend just activating a .Changed event on the AI's humanoid to see when they are shot at or hurt and then search for the closest area of cover. You can do this by naming all parts in Workspace that can be used as cover something specific and then simply use magnitude to find out which one is the closest and navigate to it using PFS. Here's an example.

function findCover()
	local ClosestDistance = 10000000
	local ClosestObject
	for _,a in pairs(workspace:GetChildren()) do
		if (a.Name == 'Cover') then
			local dis = (robot.Torso.Position - a.Position).magnitude
			if (dis < ClosestDistance) then
				ClosestObject = a
				ClosestDistance = dis
			end
		end
	end
	return ClosestObject
end

robot.Humanoid.Changed:Connect(function(prop)
	if (prop == 'Health') then
		local cover = findCover()
		-- get to the chopper!!
	end
end)

Of course, you'll need to include a check to determine when combat is over or the threat is gone and then proceed as normally.