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.go

@@ -305,20 +305,55 @@ func (t *Tender) GetMainOrganization() *Organization {
 
 // HasEstimatedValue returns true if the tender has an estimated value
 func (t *Tender) HasEstimatedValue() bool {
-	return t.EstimatedValue > 0
+	value, _ := t.ResolvedEstimatedValueAndCurrency()
+	return value > 0
+}
+
+// AggregateProcurementLotEstimatedValue sums lot-level estimated values and returns the first lot currency found.
+func AggregateProcurementLotEstimatedValue(lots []ProcurementLot) (value float64, currency string) {
+	for _, lot := range lots {
+		if lot.EstimatedValue <= 0 {
+			continue
+		}
+		value += lot.EstimatedValue
+		if currency == "" {
+			currency = strings.TrimSpace(lot.Currency)
+		}
+	}
+	return value, currency
+}
+
+// ResolvedEstimatedValueAndCurrency returns the tender-level estimated value when set,
+// otherwise aggregates procurement lot values for API and display use.
+func (t *Tender) ResolvedEstimatedValueAndCurrency() (float64, string) {
+	if t == nil {
+		return 0, ""
+	}
+	if t.EstimatedValue > 0 {
+		return t.EstimatedValue, strings.TrimSpace(t.Currency)
+	}
+	value, currency := AggregateProcurementLotEstimatedValue(t.ProcurementLots)
+	if value > 0 {
+		if currency == "" {
+			currency = strings.TrimSpace(t.Currency)
+		}
+		return value, currency
+	}
+	return 0, strings.TrimSpace(t.Currency)
 }
 
 // GetFormattedEstimatedValue returns formatted estimated value with currency
 func (t *Tender) GetFormattedEstimatedValue() string {
-	if !t.HasEstimatedValue() {
+	value, currency := t.ResolvedEstimatedValueAndCurrency()
+	if value <= 0 {
 		return ""
 	}
 
-	if t.Currency != "" {
-		return fmt.Sprintf("%.2f %s", t.EstimatedValue, t.Currency)
+	if currency != "" {
+		return fmt.Sprintf("%.2f %s", value, currency)
 	}
 
-	return fmt.Sprintf("%.2f", t.EstimatedValue)
+	return fmt.Sprintf("%.2f", value)
 }
 
 // UpdateStatus updates the tender status and updated timestamp