Can I cook Foster Farms chicken sous-vide in its original plastic?
The only way to be completely sure is to contact Foster Farms and ask them what their packaging is made of and whether or not it is a totally airtight seal.  Everything else is speculation.
There are, however, two data points on which to speculate, both from their preparation page, and this is more broadly applicable to any company that distributes food this way:

Can I freeze chicken in its original wrapper?
Yes, the original packaging is fine for freezing up to two months. For longer freezing, over-wrap packages with foil, plastic wrap, freezer paper or plastic bags.

Generally something that's been properly vacuum-sealed will last much longer than that.  The above would seem to indicate that it is not completely airtight and may even allow a small amount of moisture to get through.

What's the best method for thawing frozen chicken or turkey?
[...] You can also defrost in your microwave following the manufacturer's instructions. Remove chicken from the original package, and place on a microwave safe dish to defrost.

(emphasis mine)
This pretty clearly indicates that it is not microwave-safe, and proper vacuum bags that are safe for sous-vide are generally also microwave safe (even the cheapo FoodSaver products claim that you can boil, steam, or microwave in the bag).
So I am leaning quite heavily toward no - I don't think that the original packaging is safe for sous-vide, given the various other statements made by the company.  But again, the best way to know for sure is to contact them directly.