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.go

@@ -309,18 +309,37 @@ func (t *Tender) HasEstimatedValue() bool {
 	return value > 0
 }
 
-// AggregateProcurementLotEstimatedValue sums lot-level estimated values and returns the first lot currency found.
+// AggregateProcurementLotEstimatedValue sums lot-level estimated values when all non-empty
+// lot currencies match. Conflicting currencies are not aggregated.
 func AggregateProcurementLotEstimatedValue(lots []ProcurementLot) (value float64, currency string) {
+	var refCurrency string
+	for _, lot := range lots {
+		if lot.EstimatedValue <= 0 {
+			continue
+		}
+		c := strings.TrimSpace(lot.Currency)
+		if c == "" {
+			continue
+		}
+		if refCurrency == "" {
+			refCurrency = c
+			continue
+		}
+		if c != refCurrency {
+			return 0, ""
+		}
+	}
+
 	for _, lot := range lots {
 		if lot.EstimatedValue <= 0 {
 			continue
 		}
 		value += lot.EstimatedValue
-		if currency == "" {
-			currency = strings.TrimSpace(lot.Currency)
-		}
 	}
-	return value, currency
+	if value <= 0 {
+		return 0, ""
+	}
+	return value, refCurrency
 }
 
 // ResolvedEstimatedValueAndCurrency returns the tender-level estimated value when set,

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",