Brian Steinberg Senior TV EditorBefore finalizing Discovery’s purchase of WarnerMedia from AT&T, David Zaslav spent months holding friendly conversations with Hollywood moguls and influential agents. Now he’s locked in contentious discussions with Madison Avenue.Warner Bros. Discovery is making what media buyers and other executives say are severe demands on advertisers in the course of discussions around the industry’s annual “upfront” market, when TV networks try to sell the bulk of their advertising inventory.