It's not necessarily a "food tour" as such, but San Francisco City Guides offers a free one-hour walking tour of the Ferry Building: Ferry Building Tour- San Francisco City Guides (sfcityguides.org)

You could also just walk around the Ferry Building on your own, buying food here and there. Some places have takeout-only food that you can buy and find a place to sit either inside or outside of the building to eat it. Other places are sit-down restaurants, and some places offer both takeout and sit-down.

Here's a guide to the Ferry Building itself: Home - Ferry Building Marketplace
In addition to its regular food purveyors, there is a Farmers Market on Tue/Thu/Sat from morning to early afternoon.