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/model.go

@@ -2003,7 +2003,7 @@ func (cn ContractNotice) GetBudget() (amount string, currency string) {
 }
 
 func budgetFromProcurementLots(lots []ProcurementProjectLot) (amount string, currency string) {
-	var total float64
+	amounts := make([]valuedAmount, 0, len(lots))
 	for i := range lots {
 		pp := lots[i].ProcurementProject
 		if pp == nil || pp.RequestedTenderTotal == nil {
@@ -2017,11 +2017,10 @@ func budgetFromProcurementLots(lots []ProcurementProjectLot) (amount string, cur
 		if !ok || value <= 0 {
 			continue
 		}
-		total += value
-		if currency == "" {
-			currency = lotCurrency
-		}
+		amounts = append(amounts, valuedAmount{value: value, currency: lotCurrency})
 	}
+
+	total, currency := aggregateSameCurrencyAmounts(amounts)
 	if total <= 0 {
 		return "", ""
 	}