A 72-year-old woman from Mumbai with a history of esophageal carcinoma undergoes esophagectomy with reconstruction. Intraoperatively, the surgeon identifies the esophagus passing through the posterior mediastinum at the level of the 10th thoracic vertebra. Two hours postoperatively, the patient develops severe dysphagia and regurgitation of food. Which anatomical structure was most likely injured, and what is the embryological origin of the opening through which the esophagus normally passes?