fix: Message => str (#25876)

api/controllers/web/message.py

             questions = MessageService.get_suggested_questions_after_answer(
                 app_model=app_model, user=end_user, message_id=message_id, invoke_from=InvokeFrom.WEB_APP
             )
+            # questions is a list of strings, not a list of Message objects
+            # so we can directly return it
         except MessageNotExistsError:
             raise NotFound("Message not found")
         except ConversationNotExistsError:

api/services/message_service.py

     @classmethod
     def get_suggested_questions_after_answer(
         cls, app_model: App, user: Union[Account, EndUser] | None, message_id: str, invoke_from: InvokeFrom
-    ) -> list[Message]:
+    ) -> list[str]:
         if not user:
             raise ValueError("user cannot be None")
 
         )
 
         with measure_time() as timer:
-            questions: list[Message] = LLMGenerator.generate_suggested_questions_after_answer(
+            questions: list[str] = LLMGenerator.generate_suggested_questions_after_answer(
                 tenant_id=app_model.tenant_id, histories=histories
             )