fix: add required depends_on_stock field to stock_available XML body

PS WebService PUT /stock_availables requires depends_on_stock even when
updating only the quantity. Hard-coded to 0 (does not depend on warehouse).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sync-service/src/main/java/com/teterialosjuanjos/tfg/sync_service/integration/prestashop/PrestashopClient.java

@@ -255,6 +255,7 @@ public class PrestashopClient {
         String idAttr = dto.idProductAttribute() != null ? dto.idProductAttribute() : "0";
         sb.append("<id_product_attribute>").append(idAttr).append("</id_product_attribute>");
         sb.append("<quantity>").append(dto.quantity()).append("</quantity>");
+        sb.append("<depends_on_stock>0</depends_on_stock>");
         sb.append("</stock_available>");
         sb.append("</prestashop>");
         return sb.toString();