Brian Steinberg Senior TV EditorWarnerMedia said it posted a solid upfront, but, like its rivals this year, offered little evidence of whether it had notched volume gains in a market where advertisers were forced to capitulate to significant hikes in the rates they pay to reach large groups of viewers.In a statement, the AT&T-owned company said it had completed “the most successful Upfront marketplace in the company’s history,” in terms of securing advance ad commitments.