A 28-year-old woman presents with a 3-month history of progressive dyspnea, chest pain, and a dry cough. She reports no fever or weight loss. On examination, she has mild dyspnea at rest and reduced breath sounds bilaterally. Chest X-ray shows a large anterior mediastinal mass with displacement of the trachea to the right and compression of the left main bronchus. CT chest reveals a heterogeneous mass with areas of necrosis and hemorrhage. Serum beta-hCG and alpha-fetoprotein are both elevated. What is the most likely diagnosis?