A 7-year-old intact male German Shepherd presents for progressive pelvic limb ataxia. Myelography reveals an extradural compressive lesion at L7-S1 with a 'golf tee' sign on the lateral view. CT confirms a lateralized, calcified mass with a 'popcorn' mineralization pattern within the intervertebral foramen. Which diagnosis is most consistent with this imaging constellation?

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