feat(integration): Improve folder service and account handling
- Add file support to FolderItem schema (item_type, size, modified, extension)
- Add SharePoint-specific parameters (site_url, library_name) to folder listing
- Improve n8n webhook response handling (empty responses, non-JSON, error messages)
- Support both array and object response formats from n8n
- Fix extra_metadata serialization to JSON string in UserAccountService