(Bloomberg) -- Trading volume on Polymarket has declined for the first time in eight months, a setback that comes as its founder acknowledges missteps and its chief rival Kalshi Inc. continues to grow. Most Read from BloombergAmbani’s Cola War With Coke, Pepsi Spurs Fridge Bonanza in IndiaNvidia’s CEO Joins Trump in China With AI in the SpotlightMamdani Scraps Property Tax Hike, Counts Second-Home RevenueInside a Year of Chaos and Conflict at Kevin Hart’s Media CompanyMonthly notional trading vo