With lawmen hot on her tail, a runaway German shepherd led authorities on a harrowing highway chase down Interstate 495 yesterday afternoon, tying up traffic before she was lured into a state police cruiser with the promise of a tasty fruit-and-nut granola bar.When 7-year-old Lexi finally gave up on her four-legged getaway and agreed to be peacefully “taken into custody,” trooper Malachy Higgins said the huffing pooch leapt into the driver’s seat of his Ford Explorer, spilling his coffee and switching on the siren.