Preheat oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup. In a large mixing bowl, combine the cubed chicken, red onion, red bell pepper, green bell pepper, and zucchini.

Drizzle the olive oil over the chicken and vegetables. Add the dried oregano, garlic powder, paprika, salt, and black pepper. Toss well to ensure everything is evenly coated.

Spread the seasoned chicken and vegetables in a single layer on the prepared baking sheet. Roast for 20-25 minutes, or until the chicken is cooked through (internal temperature of 165°F / 74°C) and the vegetables are tender-crisp. You can stir halfway through cooking for even browning.

While the chicken and vegetables are roasting, prepare the herby ranch. In a small mixing bowl, whisk together the mayonnaise, sour cream (or Greek yogurt), fresh dill, chives, parsley, minced garlic, lemon juice, milk, salt, and pepper until smooth and well combined. Taste and adjust seasoning if needed. If it's too thick, add a tiny bit more milk.

Once the chicken and vegetables are cooked, remove the baking sheet from the oven. Briefly warm the pita breads according to package instructions (you can do this in the microwave, toaster, or wrap in foil and place in the oven for the last 5 minutes of cooking).

To assemble, open each warm pita and fill generously with the roasted chicken and vegetables. Top with halved cherry tomatoes and a sprinkle of crumbled feta cheese (if using). Drizzle generously with the fresh herby ranch dressing. Garnish with extra fresh parsley or dill, if desired, and serve immediately.