FIX: Allowlist topic custom field used by AI Bot (#250)

lib/modules/ai_bot/entry_point.rb

@@ -125,6 +125,10 @@ module DiscourseAi
             end
           end
         end
+
+        if plugin.respond_to?(:register_editable_topic_custom_field)
+          plugin.register_editable_topic_custom_field(:ai_persona)
+        end
       end
     end
   end