UNION CITY — Fortinet has widened its shopping spree for Bay Area real estate with the purchase of a Union City building, sending the value of its real estate acquisitions in the nine-county region past a half-billion dollars. The tech company’s property-buying expeditions are even larger in the South Bay than they are in Union City. All told, Fortinet has paid $535.7 million to buy properties in Sunnyvale, Santa Clara and Union City. In Alameda County, Fortinet, a cybersecurity company, has spent an eyebrow-raising $209 million to buy several commercial or industrial properties in Union City, according to a review…