PHILADELPHIA — The legal fight for every single vote in Pennsylvania’s Republican Senate primary has begun. Armies of attorneys representing David McCormick and Mehmet Oz, separated by a razor-thin margin in the ongoing vote count, had been descending on the state, when a federal appeals court jolted the process Friday. Shortly after 4 p.m., the court ruled that undated mail ballots from Lehigh County in last fall’s election should be counted. That sent counties and campaigns scrambling because state courts had previously held that Pennsylvania law required voters to date their ballots or have...