Enhance tender estimated value resolution and add unit tests

- Updated the `ResolvedEstimatedValueAndCurrency` method to aggregate procurement lot values when the tender-level estimated value is not set, improving accuracy in value retrieval.
- Introduced the `AggregateProcurementLotEstimatedValue` function to sum estimated values from procurement lots and return the first found currency.
- Modified the `ToResponseWithLanguage` method to utilize the new estimated value resolution logic.
- Added unit tests for the new functionality, ensuring correct behavior for various scenarios in the `entity_test.go` and `budget_test.go` files.

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

internal/tender/entity_test.go

@@ -0,0 +1,51 @@
+package tender
+
+import "testing"
+
+func TestAggregateProcurementLotEstimatedValue(t *testing.T) {
+	value, currency := AggregateProcurementLotEstimatedValue([]ProcurementLot{
+		{EstimatedValue: 100000, Currency: "EUR"},
+		{EstimatedValue: 250000, Currency: "EUR"},
+	})
+	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",
+		ProcurementLots: []ProcurementLot{
+			{LotID: "LOT-0001", EstimatedValue: 500000, Currency: "EUR"},
+		},
+	}
+
+	value, currency := tender.ResolvedEstimatedValueAndCurrency()
+	if value != 500000 {
+		t.Fatalf("expected 500000, got %v", value)
+	}
+	if currency != "EUR" {
+		t.Fatalf("expected EUR, got %q", currency)
+	}
+}
+
+func TestResolvedEstimatedValueAndCurrencyPrefersTopLevel(t *testing.T) {
+	tender := &Tender{
+		EstimatedValue: 900000,
+		Currency:       "USD",
+		ProcurementLots: []ProcurementLot{
+			{EstimatedValue: 100000, Currency: "EUR"},
+		},
+	}
+
+	value, currency := tender.ResolvedEstimatedValueAndCurrency()
+	if value != 900000 {
+		t.Fatalf("expected 900000, got %v", value)
+	}
+	if currency != "USD" {
+		t.Fatalf("expected USD, got %q", currency)
+	}
+}