A 58-year-old man from Delhi presents to the emergency department with fever (39.2°C), dysuria, and flank pain for 3 days. He has a history of recurrent urinary tract infections treated with multiple antibiotics over the past year. Urinalysis shows pyuria and bacteriuria. Urine culture grows a gram-negative rod. On antibiotic susceptibility testing, the organism is resistant to ceftriaxone and cefotaxime but susceptible to carbapenems and fluoroquinolones. The ESBL confirmatory test (double-disk synergy test) is positive. Which mechanism of resistance is most likely responsible for the cephalosporin resistance in this isolate?