A 72-year-old woman with a history of small-cell lung cancer (SCLC) on active chemotherapy presents with progressive lethargy, anorexia, and a single episode of vomiting over 3 days. She denies polydipsia or polyuria. On examination, she is alert but lethargic, blood pressure 118/76 mmHg (baseline 130/80), heart rate 88/min, and mucous membranes are moist. There is no peripheral edema. Laboratory findings: serum sodium 124 mEq/L, serum osmolality 258 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 68 mEq/L, serum creatinine 0.9 mg/dL (baseline 1.0 mg/dL), TSH 2.1 mIU/L. What is the most appropriate initial management?