I would imagine that the lack of any kind of food source for bacteria, freezing cold temperatures, and space radiation should be enough to sanitize the vehicle from anything from earth. We're not talking about organisms that thrive at the depths of the ocean or tardigrades (although I'm sure someone will bring them up). We're talking about molds and bacterias that grow in warm, damp places like car seats and dirty human crevices. I would be surprised if they lasted out there and would be giddy if it survived long enough to grow further. That's an experiment, right there.

You could be surprise how resilient are the microbes against the harsh conditions of space.

There's enough places for microbes to hide from radiation. On that short of a mission it wouldn't be surprising to see microbes. There are various microbes that can withstand the nominal radiation there. Would be much harder to survive the Van Allen belts and CMEs, but it is possible. If we were talking about a 100 year mission, that would be more surprising. Even hardy microbes don't like extended stays in vacuums and have to go into a hibernation state. There are bacteria that can survive for decades in a vacuum and low temperatures.

Disclaimer: not a biologist

Some bacteria can survive very high levels of radiation. For example: https://en.wikipedia.org/wiki/Deinococcus_radiodurans