A 35-year-old patient complains of short-term pain in tooth #26 when eating cold food. Clinical examination reveals a deep cavity in the occlusal surface with a narrow entrance, filled with softened dentin. The cavity walls and floor are pigmented. Probing causes a sharp pain, cold test elicits a brief painful response that disappears immediately after the irritant is removed. Percussion is painless. What is the most likely diagnosis?