Some of our chickens have free ranged before and we have lots of plants I don't think they have ever gotten sick for that reason. We also have baby fruit trees, and they are able to reach the fruit if they hop on the lowest branch, but never have they eaten the fruit. Welcome!
I routinely throw prunings from the garden, and kitchen scraps into the run, and they always scratch around in the pile, but have never gotten sick. They love tomatoes, but I've never seen them eat the leaves.
Also, they make a wide berth around onions, avocado scraps, and citrus. My feeling is that they somehow know not to eat the questionable stuff, so it goes into the run compost pile.
I do avoid throwing moldy, or rotten stuff in there, even though it molds, and rots in the compost pile anyway !
My chickens are (very) well fed, and seem to be pretty picky about their diet, turning their little noses up at anything they deem unworthy to eat.
Welcome to Backyard Chickens, I'm pretty picky about food myself. I've read that tomato leaves are poisonous but don't know to who. Tomatoes are in the "nightshade" family which I have heard are poisonous.