feat(workflow): move conversation file content into metadata

api/app/schemas/conversation_schema.py

@@ -51,6 +51,10 @@ class Message(BaseModel):
     def _serialize_created_at(self, dt: datetime.datetime):
         return int(dt.timestamp() * 1000) if dt else None
 
+    @field_serializer("meta_data", when_used="json")
+    def _serialize_meta_data(self, data: Optional[Dict[str, Any]]):
+        return data or {}
+
 
 class Conversation(BaseModel):
     """会话输出"""