Function validate_command_string 

pub fn validate_command_string(cmd: &str) -> Result<(), NikaError>

Validate command string for control characters

Rejects control characters (0x00-0x1F) except:

• \n (0x0A) - newline, allowed for multi-line commands
• \t (0x09) - tab, allowed for indentation

Errors

Returns BlockedCommand if a control character is found.