agent: small refactoring and tests for battery package (#1872)

agent/battery/battery_darwin.go

@@ -39,15 +39,13 @@ func readMacBatteries() ([]macBattery, error) {
 var HasReadableBattery = sync.OnceValue(func() bool {
 	systemHasBattery := false
 	batteries, err := readMacBatteries()
+	slog.Debug("Batteries", "batteries", batteries, "err", err)
 	for _, bat := range batteries {
 		if bat.MaxCapacity > 0 {
 			systemHasBattery = true
 			break
 		}
 	}
-	if !systemHasBattery {
-		slog.Debug("No battery found", "err", err)
-	}
 	return systemHasBattery
 })