fix: handle PS 8.x {"products":false} response in getProductByReference

PS 8.2.5 returns {"products": false} instead of {"products": []}
when a reference filter finds no matching products. Jackson cannot
deserialize false as a List, throwing HttpMessageConversionException.
Catch it and return Optional.empty() so pushProduct takes the CREATE
path correctly.

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

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

@@ -6,6 +6,7 @@ import com.teterialosjuanjos.tfg.sync_service.integration.prestashop.exception.P
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConversionException;
 import org.springframework.stereotype.Component;
 import org.springframework.web.client.RestClient;
 
@@ -87,6 +88,10 @@ public class PrestashopClient {
             // PS may return 404 instead of 200+empty when filter finds no results
             if (e.getStatusCode() != null && e.getStatusCode().value() == 404) return Optional.empty();
             throw e;
+        } catch (HttpMessageConversionException e) {
+            // PS 8.x returns {"products": false} instead of {"products": []} when filter has no results
+            log.debug("PS returned unparseable body for reference filter={}, treating as not found", reference);
+            return Optional.empty();
         }
     }