fix: correct Dolibarr/PS filters and handle guest orders

- DolibarrClient.getProductByRef: use sqlfilters=(t.ref:=:'x') instead
  of unsupported ref query param (caused StockSync to push wrong stock)
- PrestashopClient.getProductByReference: wrap value in [x] for PS
  exact-match filter syntax (root cause of createProduct returning null)
- OrderSyncService.resolveCustomer: skip getCustomer when id_customer
  is 0 or null (guest orders returned 404 and failed import)
- StockSyncService: parse stock quantity as Double to handle "5.000"

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

sync-service/src/main/java/com/teterialosjuanjos/tfg/sync_service/integration/dolibarr/DolibarrClient.java

@@ -49,9 +49,10 @@ public class DolibarrClient {
      * @param ref SKU value, e.g. "PROD-001"
      */
     public DolibarrProductDto getProductByRef(String ref) {
+        String filter = "(t.ref:=:'%s')".formatted(ref);
         List<DolibarrProductDto> results = restClient.get()
                 .uri(u -> u.path("/products")
-                        .queryParam("ref", ref)
+                        .queryParam("sqlfilters", filter)
                         .queryParam("limit", 1)
                         .build())
                 .retrieve()

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

@@ -68,12 +68,14 @@ public class PrestashopClient {
      * @param reference SKU value, e.g. "PROD-001"
      */
     public Optional<PrestashopProductDto> getProductByReference(String reference) {
+        // PS WebService requires [value] brackets for exact-match filtering
+        String exactRef = "[" + reference + "]";
         PrestashopProductDto.ListResponse response = restClient.get()
                 .uri(u -> u.path("/products")
                         .queryParam("ws_key", wsKey)
                         .queryParam("output_format", "JSON")
                         .queryParam("display", "full")
-                        .queryParam("filter[reference]", reference)
+                        .queryParam("filter[reference]", exactRef)
                         .build())
                 .retrieve()
                 .body(PrestashopProductDto.ListResponse.class);

sync-service/src/main/java/com/teterialosjuanjos/tfg/sync_service/sync/OrderSyncService.java

@@ -147,8 +147,15 @@ public class OrderSyncService {
     }
 
     private DolibarrThirdpartyDto resolveCustomer(PrestashopOrderDto psOrder) {
-        PrestashopCustomerDto psCustomer = prestashopClient.getCustomer(
-                Integer.parseInt(psOrder.idCustomer()));
+        // Guest orders have id_customer=0 — skip getCustomer call to avoid 404
+        boolean isGuest = psOrder.idCustomer() == null
+                || psOrder.idCustomer().isBlank()
+                || "0".equals(psOrder.idCustomer());
+
+        PrestashopCustomerDto psCustomer = null;
+        if (!isGuest) {
+            psCustomer = prestashopClient.getCustomer(Integer.parseInt(psOrder.idCustomer()));
+        }
 
         String name = psCustomer != null
                 ? (psCustomer.firstname() + " " + psCustomer.lastname()).trim()

sync-service/src/main/java/com/teterialosjuanjos/tfg/sync_service/sync/StockSyncService.java

@@ -106,7 +106,7 @@ public class StockSyncService {
 
         PrestashopStockAvailableDto current = stockOpt.get();
         int targetQty = dolProduct.stockReel().intValue();
-        int currentQty = Integer.parseInt(current.quantity());
+        int currentQty = (int) Double.parseDouble(current.quantity());
 
         if (targetQty == currentQty) {
             return false;