Rats can attack chickens, but most likely won't. They would most likely be more inclined to rob eggs or the food bowl. A sick or injured bird, of any size is at risk from rats, healthy ones that fight back can usually defend themselves. To reduce the possibility of rats in the coop, keep feed out of roosting areas, chickens won't starve overnight so leave the food out in the run. Collect any eggs regularly and make sure they aren't left inside overnight. Don't store extra feed or treats in the coop. Rats proofing a coop can be done with the methods described by others here. But to me there are other predators that pose a higher risk that need attention first.