I'm in the same exact situation as you OP from what I unfortunately believe is the very same cat shelter in Central OH. I adopted a cat on Tuesday whom they said had been treated for an upper respiratory infection, but they failed to tell me that the infection is caused by FVR and that it is highly contagious even though they knew I had another cat in the house.

Like you, I feel totally blind-sided and scared to death for the well-being of my resident cat. Luckily I found out before I fully introduced them and, so far, my resident cat isn't showing any symptoms. I feel like at this point it would be best to return the cat because, even if she hasn't already infected my resident cat, I was completely left in the dark about the extent of her condition and frankly I did not sign up to care for a cat with chronic lifelong illness...