Function validate_filename 

pub fn validate_filename(
    filename: &str,
) -> Result<(), Box<dyn Error + Send + Sync>>

Validate that a filename is safe and valid.

Errors

Returns an error if the filename:

• Is empty
• Contains directory separators
• Contains null bytes
• Contains path traversal sequences (..)
• Exceeds 255 bytes
• Matches a Windows reserved name (CON, NUL, COM1–9, LPT1–9, etc.)