fixed minor access bug

Core/src/main/java/me/neurodock/backend/open/ai/OpenAIAssistentMessage.java

@@ -25,7 +25,7 @@ public class OpenAIAssistentMessage extends Message {
         return endReason;
     }
 
-    enum Reason {
+    public static enum Reason {
         STOP,
         LENGTH,
         TOOL_CALLS,