A 28-year-old woman presents with a 6-month history of progressive headaches, morning vomiting, and recent-onset diplopia. On examination, she has papilledema and a sixth nerve palsy. MRI brain with contrast shows a well-circumscribed, homogeneously enhancing mass in the pineal region, measuring 2.5 cm, with obstructive hydrocephalus. There is no restricted diffusion or hemorrhage. What is the most likely diagnosis?