SPRINGFIELD — A union lobbyist who qualified for a public schoolteacher’s pension by substitute teaching for one day can keep his benefits, the Illinois Supreme Court ruled Thursday.The Illinois Constitution prohibits reversing public pension benefits that were previously promised under the pension-protection clause, the court’s 4-3 majority found, even if lawmakers chose to close off such a loophole for others in the future.Justice Anne M. Burke delivered the majority opinion, joined by Chief Justice …