DETROIT — A judge declared a mistrial Thursday after jurors said they couldn’t reach a verdict in a dispute over whether two engineering firms should bear some responsibility for Flint’s lead-contaminated water.Veolia North America and Lockwood, Andrews & Newnam, known as LAN, were accused of not doing enough to get Flint to treat the highly corrosive water or to urge a return to a regional water supplier.A mistrial was declared in federal court in Ann Arbor, Michigan, court spokesman David …