FTP and SFTP steps add timeouts, retries, and reusable downloads

FTP and SFTP workflow steps now handle common file operations with better timeout, retry, and cleanup behavior.

• Download file content in a format that can be passed to later workflow steps.
• Retry uploads after temporary failures.
• Time out list, upload, download, rename, and close operations instead of hanging indefinitely.
• Report file operation errors more consistently.