Is it possible for a cadet to ensure that he/she commissions as a Sapper if they choose engineering as their branch?

Sapper is a job or additional school, not an MOS or AOC. Not all combat engineers are sappers. As a cadet you can't even guarantee you'll be an engineer. But you don't have to be an engineer to go to sapper school. And other MOS/AOCs do sapper like things without being sappers or engineers. I was a maintenence platoon leader, ordance LT with no special schools, in a heavy engineer company the first time i built and fired a Bangalore torpedo.

You’re not guaranteed Engineer unless you’re going guard or reserve and have the paragraph and line number before you graduate. Then it is up to your unit to send you if you’re in a combat engineer unit.

All I can say after working with a few engineers over the years is it is very selective. Being prepared mentally and physically would probably be your best bet to being selected.