Enhance procurement lot estimated value aggregation and add unit tests

- Updated the `AggregateProcurementLotEstimatedValue` function to reject mixed currencies and allow empty lot currencies, ensuring accurate aggregation of estimated values.
- Introduced new unit tests in `entity_test.go` to validate the behavior of the aggregation function under various scenarios, including mixed currencies and empty lot currencies.
- Refactored budget calculation in `budget_test.go` to utilize the new aggregation logic, improving consistency in budget retrieval.

This update improves the handling of estimated values in procurement lots, enhancing the reliability of the tender management system.

internal/tender/entity_test.go

@@ -15,6 +15,29 @@ func TestAggregateProcurementLotEstimatedValue(t *testing.T) {
 	}
 }
 
+func TestAggregateProcurementLotEstimatedValueRejectsMixedCurrencies(t *testing.T) {
+	value, currency := AggregateProcurementLotEstimatedValue([]ProcurementLot{
+		{EstimatedValue: 100000, Currency: "EUR"},
+		{EstimatedValue: 250000, Currency: "USD"},
+	})
+	if value != 0 || currency != "" {
+		t.Fatalf("expected no aggregation for mixed currencies, got value=%v currency=%q", value, currency)
+	}
+}
+
+func TestAggregateProcurementLotEstimatedValueAllowsEmptyLotCurrency(t *testing.T) {
+	value, currency := AggregateProcurementLotEstimatedValue([]ProcurementLot{
+		{EstimatedValue: 100000, Currency: "EUR"},
+		{EstimatedValue: 250000},
+	})
+	if value != 350000 {
+		t.Fatalf("expected total 350000, got %v", value)
+	}
+	if currency != "EUR" {
+		t.Fatalf("expected currency EUR, got %q", currency)
+	}
+}
+
 func TestResolvedEstimatedValueAndCurrencyUsesLotsWhenTopLevelMissing(t *testing.T) {
 	tender := &Tender{
 		Currency: "EUR",