Smart, sexy, funny… “Burn Notice” has been the perfect summer TV cocktail. A pure pleasure. The action-adventure series about a tarnished spy in Miami — with charismatic Jeffrey Donovan heading an excellent cast — has been a smash hit for USA Network, if you haven’t noticed. “Characters welcome” is USA’s slogan, and Donovan’s Michael Westen fits the bill. “Burn Notice” wraps up its first season with a two-hour episode Thursday at 9 p.m. Time to take a sip if you haven’t already.