NEOADJUVANT TREATMENT (RE-INITIATED): Patients receive pembrolizumab IV over 30 minutes on days 1 and 22 and oxaliplatin IV over 2-6 hours, leucovorin calcium IV over 15 minutes to 2 hours, and fluorouracil IV over 46-48 hours on days 1, 15, and 29. Treatment repeats every 21 days for up to 4 courses in the absence of disease progression or unacceptable toxicity.