runner catches ball while in foul territory

[bob jenkins]

If the ball had a chance to go fair, it's INT. If not, it's a foul ball.

Based on your description, the latter applies.

Here's the OBR rule -- it's black and white, but the interp is as I mentioned above w.r.t. having a chance to go fair:

(2) He intentionally deflects the course of a foul ball in any
manner;