A 2-year-old girl presents with a small amount of blood-tinged vaginal spotting and is found to have a soft, pedunculated, mobile mass protruding from the vaginal introitus. Examination under anesthesia with excisional biopsy is performed. The mass is well-circumscribed, covered by benign epithelium, and shows mature fibrovascular stroma without cytologic atypia or invasive growth on histology. What is the most likely diagnosis?