Welcome to the forum.
Yes, that bolt comes with a new OEM tensioner, same with idler pulley bolt.
Ford Dealer won't have it listed as a separate part so can't
tell you the length or even the size because they don't have it listed anywhere, no won't involved.
Best bet is to measure it, tensioner thickness and hole depth, then hit the parts store.
If you can get the parts store guy to pull out a Motorcraft(OEM) tensioner it should have the bolt with it, measure it to get replacement.
my guess would be a 10mm bolt(size not length), needs to be harder grade because these are known to break, which is why you get a new bolt with OEM tensioner
Found this diagram: http://www.justanswer.com/uploads/lo...1_213122_2.gif
This is what the Ford parts guy sees, you can see the tensioner, with bolt installed(same with idler pulley) and no separate part designation to look up like the other bolts shown