Add remaining 1/2 tablespoon oil, swirling to coat sides. Add garlic and cook, stirring, until fragrant, about 10 seconds. Add celery, bell pepper, and bamboo shoots; stir-fry for 1 1/2 minutes. Return chicken and chilies to wok; stir-fry for 1 minute. Add sauce and bring to a boil. Add cornstarch solution and cook, stirring, until sauce boils and thickens. Add walnuts and toss to coat.