Preheat oven to 350 degrees. Prepare a 9x13 baking dish with nonstick cooking spray. In a large skillet over medium-high heat, place the butter to melt. Add the celery and onion and cook, stirring often, until softened. To the skillet, add the chicken stock, cream of chicken soup, salt, poultry seasoning, ground sage, pepper, and garlic. Stir to combine and remove the skillet from heat. Place the stuffing mix, cornbread, chicken, and the liquid mixture in a large bowl. Gently fold the mixture together until the cornbread is moist. Transfer the mix to the prepared baking dish. Bake for 45 minutes or until the edges are browned and crunchy but the middle is cooked through.