chore: rename StartReturningModel to Run

Added aliases for the old functions, but deprecated them.

examples/send-msg/main.go

@@ -117,7 +117,7 @@ func main() {
 		}
 	}()
 
-	if err := p.Start(); err != nil {
+	if _, err := p.Run(); err != nil {
 		fmt.Println("Error running program:", err)
 		os.Exit(1)
 	}