To prepare kebabs, combine the first 12 ingredients in a large zip-top plastic bag; seal and marinate in refrigerator overnight, turning bag occasionally.
Remove chicken from bag; discard marinade. Thread chicken, onion, bell pepper, and zucchini alternately on each of 8 (12-inch) wooden skewers. Coat kebabs with cooking spray, and sprinkle with 1/2 teaspoon salt and black pepper. Place kebabs on grill rack coated with cooking spray. Grill 25 minutes or until chicken is done, turning occasionally. Remove from grill; keep warm.
To prepare raita, combine 1/2 cup yogurt and remaining ingredients in a small bowl. Serve with kebabs.