A 2-year-old girl from Delhi presents with a 2-month history of progressive abdominal distension and irritability. Her mother reports that she has lost weight and has had intermittent fever. On examination, the child is pale and has a firm, fixed mass in the left upper abdomen that crosses the midline. Laboratory investigations show hemoglobin 8.5 g/dL, platelet count 450,000/μL, and elevated 24-hour urine vanillylmandelic acid (VMA). Chest X-ray reveals a left-sided mediastinal mass. What is the most likely diagnosis?