Make vendor nullable

internal/validator/purchase_order_validator_test.go

@@ -11,12 +11,13 @@ import (
 )
 
 func validCreatePurchaseOrderRequest() *contract.CreatePurchaseOrderRequest {
+	vendorID := uuid.New()
 	ingredientID := uuid.New()
 	quantity := 1.0
 	unitID := uuid.New()
 
 	return &contract.CreatePurchaseOrderRequest{
-		VendorID:        uuid.New(),
+		VendorID:        &vendorID,
 		PONumber:        "PO-001",
 		TransactionDate: "2026-05-29",
 		Items: []contract.CreatePurchaseOrderItemRequest{
@@ -31,6 +32,30 @@ func validCreatePurchaseOrderRequest() *contract.CreatePurchaseOrderRequest {
 	}
 }
 
+func TestPurchaseOrderValidatorCreateAllowsMissingVendor(t *testing.T) {
+	validator := NewPurchaseOrderValidator()
+	req := validCreatePurchaseOrderRequest()
+	req.VendorID = nil
+
+	err, code := validator.ValidateCreatePurchaseOrderRequest(req)
+
+	require.NoError(t, err)
+	require.Empty(t, code)
+}
+
+func TestPurchaseOrderValidatorCreateRejectsEmptyVendor(t *testing.T) {
+	validator := NewPurchaseOrderValidator()
+	req := validCreatePurchaseOrderRequest()
+	vendorID := uuid.Nil
+	req.VendorID = &vendorID
+
+	err, code := validator.ValidateCreatePurchaseOrderRequest(req)
+
+	require.Error(t, err)
+	require.Equal(t, constants.MalformedFieldErrorCode, code)
+	require.Contains(t, err.Error(), "vendor_id cannot be empty")
+}
+
 func TestPurchaseOrderValidatorCreateAllowsMissingDueDate(t *testing.T) {
 	validator := NewPurchaseOrderValidator()