My feeling is that foraging chickens have a choice in what they eat and will not eat anything that is bad for them. In my yard where the chickens forage daily, there are plants that are poisonous for them. They don't eat them.
Watching them forage, I see that they spend most of their time digging for bugs and such as well as nibbling from the grass and weeds. They seem to be specific in what they eat.
I have a large sage plant. They will eat some of the flowers off of it at times, but not all of them. They don't eat the leaves. Considering how strong the smell of sage can be, I don't blame them. Nevertheless, the chickens smell nice after they have been rummaging around in the sage.
Chris