fix: resolve compilation errors (missing log import and package conflicts)

backend/internal/infrastructure/payments/mercadopago.go

@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"net/http"
 	"strconv"
-	"strings"
 
 	"github.com/saveinmed/backend-go/internal/domain"
 )

backend/internal/infrastructure/payments/split_math_test.go

@@ -1,4 +1,4 @@
-package tests
+package payments
 
 import (
 	"math"

backend/internal/usecase/shipping_usecase.go

@@ -3,6 +3,7 @@ package usecase
 import (
 	"context"
 	"errors"
+	"fmt"
 	"math"
 
 	"github.com/gofrs/uuid/v5"

backend/internal/usecase/usecase.go

@@ -2,6 +2,7 @@ package usecase
 
 import (
 	"context"
+	"log"
 	"time"
 
 	"github.com/gofrs/uuid/v5"