My guess: The contractor is going to say that's a hairline crack, they happen. He's going to refuse to accept that the crack is because he didn't do the job correctly. I'm not saying he did the job incorrectly either (i don't have expansion joints in my corners, 1 is cracked! Perhaps my concrete guy didn't know any better either, i'll have to research proper location of expansion joints).
I told my concrete guy that i didn't want any cracks. I asked what could we do to prevent it. He said I didn't want to pay him what it would cost him to build it such that he could guarantee any cracks would only fall in expansion joints. (and of course he said he wouldn't guarantee it even then). I've got three hairline cracks not at expansion joints. They have not worsened in the year since the pour.
The only real solution is to cut out areas and re-pour. Consequences of that are that you end up with new joints that may look out of place.
One of the various concrete coatings could be used, but typically cracks will transfer through them to the surface as well.