Wood siding meets concrete
Wood meeting concrete in a damp area is never a good recipe. As porous as concrete is, its not even ideal that treated wood meets concrete. this is mostly because of the chance for standing water or leeching.

My recommendation in this scenario would be to trim the wood siding ~1/4 inch minimally, or better yet 6" to 8" above the concrete area. If you go inches, (which you should) install aluminum flashing (for example).

After doing the trimming, you should use flashing or caulk as you suggested to not so much as seal it off, but while sealing it off, make a curved bead that directs water away from the wood.

I would have thought ideally the contractor would have put expansion strips here right along the side, and ideally should have discussed with you the height of the concrete once cured and what the plans were for water control and keeping it off of the structure.

What you have there now is going to rot, caulking it would slow it down from the outside, but underneath it would probably accelerate it since the moisture will wick up behind the caulk and up the siding.

Here is a quick google search of water wicking up wood:
http://www.cwfbuilders.com/images/Water_damage_at_walls_from_patio.JPG

Here is just one of many views yours could look like with flashing. Just picture it is a slab instead of a deck, Also, consider direction of travel for water. In this photo while it gives you and idea, the trim strip is not something you'd install on top, this is just a pictoral reference form anotehr quick google:
http://decks.blob.core.windows.net/img/articles/large/16011219304064.jpg