A 28-year-old man presents with a painless testicular mass. On gross examination, the tumor is well-circumscribed, homogeneous, and gray-white, with a soft, fleshy appearance and minimal hemorrhage or necrosis. Histological examination shows sheets and nests of uniform polygonal cells with abundant clear cytoplasm containing glycogen and prominent nucleoli, separated by delicate fibrous septa infiltrated by lymphocytes. Immunohistochemistry shows tumor cells that are positive for CD117 (c-KIT) and OCT3/4. What is the most likely diagnosis?