SOFIA — Bulgaria’s former president and air force chief, Rumen Radev, is on course to win a general election on Sunday with promises to combat the all-pervasive “mafia state” that he accuses of undermining the EU’s poorest country. Roiled by incessant political crises and weakened by fragile coalitions, the Balkan nation of 6.7 million is becoming close to ungovernable. It has had seven prime ministers since 2021 — none of them serving a full term — and this weekend’s vote will be the eighth