A 62-year-old woman with type 2 diabetes mellitus on metformin and glibenclamide presents with a 5-day history of progressive confusion, weakness, and polyuria. On examination, she is drowsy, respiratory rate 16/min (not labored), blood pressure 95/60 mmHg, and heart rate 112/min. Capillary blood glucose is 680 mg/dL. Arterial blood gas shows pH 7.32, HCO₃⁻ 18 mEq/L, and PaCO₂ 38 mmHg. Serum osmolality is 335 mOsm/kg. Serum sodium is 125 mEq/L (corrected). Urine ketones are negative. What is the most appropriate immediate next step in management?