Halfway through his second round of batting practice with the Mets before Saturday’s 3-2 win over the Nationals, Yoenis Cespedes launched a home run several rows deep into the left-center-field seats — a place where Major League Baseball players don’t typically hit baseballs. Instantly, he validated the hopes of manager Terry Collins, who was sitting in the dugout at the time, laughing and joking in a grand old mood.