Function parse_optional_value_flag 

pub fn parse_optional_value_flag(
    args: &[String],
    command: &str,
    flag: &str,
) -> Result<Option<String>, ArgParseError>

Scan for an optional --flag value pair. Returns Ok(None) if the flag is absent, Ok(Some(value)) if present with a value, and an error for duplicates, missing values, or unexpected trailing arguments.