UPN Entertainment President Dawn Ostroff rose to her biggest challenge last year, successfully filling “Buffy the Vampire Slayer’s” shoes and shoring up Tuesday night with the network’s first successful reality show, “America’s Next Top Model.” Four of UPN’s five new shows were picked up for a full season, and freshman sitcom “Eve” is now the network’s most-watched show with 4.1 million total viewers. Ms. Ostroff also made good on her promise to bring A-list talent to the network, with Will Smith and Jada Pinkett-Smith executive producing “All of Us” and Samuel L. Jackson, Mel Gibson and Mimi Leder developing projects for next season.