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.

ted/budget_test.go

@@ -56,3 +56,29 @@ func TestContractNoticeGetBudgetSumsAllLots(t *testing.T) {
 		t.Fatalf("expected EUR, got %q", currency)
 	}
 }
+
+func TestContractNoticeGetBudgetRejectsMixedCurrencies(t *testing.T) {
+	cn := ContractNotice{
+		ProcurementProjectLots: []ProcurementProjectLot{
+			{
+				ProcurementProject: &ProcurementProject{
+					RequestedTenderTotal: &RequestedTenderTotal{
+						EstimatedOverallContractAmount: CurrencyText{CurrencyID: "EUR", Text: "100000"},
+					},
+				},
+			},
+			{
+				ProcurementProject: &ProcurementProject{
+					RequestedTenderTotal: &RequestedTenderTotal{
+						EstimatedOverallContractAmount: CurrencyText{CurrencyID: "USD", Text: "250000"},
+					},
+				},
+			},
+		},
+	}
+
+	amount, currency := cn.GetBudget()
+	if amount != "" || currency != "" {
+		t.Fatalf("expected no budget for mixed currencies, got amount=%q currency=%q", amount, currency)
+	}
+}