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/form.go

@@ -277,6 +277,8 @@ func (t *Tender) ToResponseWithLanguage(language string) *TenderResponse {
 		documentURL = strings.TrimSpace(t.TenderURL)
 	}
 
+	estimatedValue, currency := t.ResolvedEstimatedValueAndCurrency()
+
 	response := &TenderResponse{
 		ID:                          t.ID.Hex(),
 		NoticePublicationID:         t.NoticePublicationID,
@@ -307,8 +309,8 @@ func (t *Tender) ToResponseWithLanguage(language string) *TenderResponse {
 		MainClassificationDescription: mainDesc,
 		MainClassificationDisplay:     classificationDisplay(mainCode, mainDesc),
 		AdditionalClassifications:     append([]string(nil), t.AdditionalClassifications...),
-		EstimatedValue:                t.EstimatedValue,
-		Currency:                      strings.TrimSpace(t.Currency),
+		EstimatedValue:                estimatedValue,
+		Currency:                      currency,
 		EstimatedValueVATBasis:        vatBasisUnspecified,
 
 		Duration:     t.Duration,