fix: [PIDM-1487] add missing cbill set to update mapper

src/main/java/it/pagopa/selfcare/pagopa/backoffice/mapper/CreditorInstitutionMapperImpl.java

@@ -154,6 +154,7 @@ public CreditorInstitutionDetails fromDto(UpdateCreditorInstitutionDto dto) {
             creditorInstitutionDetails.setCreditorInstitutionCode(dto.getCreditorInstitutionCode());
             creditorInstitutionDetails.setEnabled(dto.getEnabled());
             creditorInstitutionDetails.setBusinessName(dto.getBusinessName());
+            creditorInstitutionDetails.setCbillCode(dto.getCbillCode());
             creditorInstitutionDetails.setAddress(fromDto(dto.getAddress()));
             creditorInstitutionDetails.setPspPayment(dto.getPspPayment());
             creditorInstitutionDetails.setReportingFtp(dto.getReportingFtp());

src/test/java/it/pagopa/selfcare/pagopa/backoffice/service/CreditorInstitutionServiceTest.java

@@ -370,6 +370,20 @@ void updateCreditorInstitution_ok() throws IOException {
         assertNotNull(result);
     }
 
+    @Test
+    void updateCreditorInstitution_shouldPreserveCbillCode() throws IOException {
+        when(apiConfigClient.updateCreditorInstitutionDetails(anyString(), any(CreditorInstitutionDetails.class)))
+                .thenAnswer(invocation -> invocation.getArgument(1));
+
+        UpdateCreditorInstitutionDto dto = TestUtil.fileToObject("request/post_creditor_institution.json", UpdateCreditorInstitutionDto.class);
+        assertEquals("COXYZ", dto.getCbillCode(), "Test fixture should have cbillCode");
+
+        CreditorInstitutionDetailsResource result = service.updateCreditorInstitutionDetails(CI_TAX_CODE, dto);
+
+        assertNotNull(result);
+        assertEquals("COXYZ", result.getCbillCode(), "cbillCode must be preserved through the update mapper");
+    }
+
     @Test
     void updateCreditorInstitution_ko() throws IOException {
         FeignException feignException = mock(FeignException.InternalServerError.class);