A 58-year-old man with a 10-year history of hypertension (currently on amlodipine and lisinopril) and type 2 diabetes mellitus (on metformin) presents for elective laparoscopic cholecystectomy. On preoperative assessment, his blood pressure is 138/86 mmHg, heart rate 78/min, and fasting blood glucose 156 mg/dL. He denies chest pain, dyspnea, or palpitations. Physical examination is unremarkable. Which investigation is most appropriate to assess his cardiac risk before surgery?