fix: throw PrestashopApiException instead of NPE when createProduct returns null

If PS accepts the POST but the subsequent GET by reference finds nothing,
the null return from createProduct now throws PrestashopApiException, which
is caught by the per-item handler and recorded in SyncLog without aborting
the rest of the sync loop.

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

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

@@ -17,6 +17,13 @@ public class PrestashopApiException extends RuntimeException {
         this.body = body;
     }
 
+    /** Use when there is no HTTP status code (e.g. inconsistent API state). */
+    public PrestashopApiException(String message) {
+        super(message);
+        this.statusCode = null;
+        this.body = null;
+    }
+
     public HttpStatusCode getStatusCode() { return statusCode; }
     public String getBody() { return body; }
 }

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

@@ -98,6 +98,10 @@ public class ProductSyncService {
 
         if (existing.isEmpty()) {
             PrestashopProductDto created = prestashopClient.createProduct(toPrestashopDto(dolProduct, null));
+            if (created == null) {
+                throw new PrestashopApiException(
+                        "Product created in PS but not found via GET (reference=" + sku + ")");
+            }
             productMappingRepository.save(ProductMapping.builder()
                     .sku(sku)
                     .dolibarrId(dolProduct.id())