fix: return null instead of error message on deserialization failure

When deserializeErrorEntity fails, return null instead of a synthetic
String message to maintain correct errorEntity semantics (null on failure).

Fixes P2 issue from cubic-dev-ai review.

Author: Chhida

modules/openapi-generator/src/main/resources/Java/libraries/jersey3/ApiClient.mustache

@@ -1369,7 +1369,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
     try {
       return deserialize(response, errorType);
     } catch (Exception e) {
-      return String.format("Failed deserializing error entity: %s", e.toString());
+      return null;
     }
   }
 

samples/client/petstore/java/jersey3/src/main/java/org/openapitools/client/ApiClient.java

@@ -1363,7 +1363,7 @@ private Object deserializeErrorEntity(Map<String, GenericType> errorTypes, Respo
     try {
       return deserialize(response, errorType);
     } catch (Exception e) {
-      return String.format("Failed deserializing error entity: %s", e.toString());
+      return null;
     }
   }