You totally did NOT ruin your saison. At a 1.012 FG, your saison will definitely benefit from some Brett C. As stated previously this strain is not a super-attenuator and works quite slowly compared to other commercially available brett strains. With that being said it is a great yeast and produces some interesting light tropical fruit notes like pineapple and mango when used in secondary.
You may or may not see a pellicle (thin white layer) on top of your saison as the brett begins to work on the beer. The pellicle typically forms in response to available oxygen in the headspace of your fermenter and works to protect the beer from other aerobic organisms giving the brett a chance to take hold in the beer. After around 1 mos you might start to see large bubbles on the surface of the beer which can sometimes lead to a "dusty" pellicle with this strain of brett.
I would at least give the beer 2 mos before taking a gravity reading (and taste of course...). At that point if the beer is down to 1.005 or lower, than I would have no problems bottling it if you like the flavor and aroma characteristics at that stage (although I don't think this brett will eat that many points in that time frame, but I could be wrong).
Be patient, try to keep the beer cool (around low 60's) for best flavor/aroma development and to hopefully avoid any overly goaty, horsey, or overly barnyard/sweaty characteristics which brett can impart.
Welcome to the crazy world of FUNK!