Function build_raw_request_frame 

pub fn build_raw_request_frame(
    opcode_byte: u8,
    opaque: u32,
    cas: u64,
    extras: &[u8],
    key: &[u8],
    value: &[u8],
) -> Vec<u8> 

Build a request frame using a raw opcode byte. Useful for testing unknown-opcode handling.