Function encode_message 

pub fn encode_message<T: Serialize>(msg: &T) -> Result<BytesMut, ProtocolError>

Serialize a message to a length-prefixed byte buffer with protocol version.

Format: [4-byte LE length][4-byte LE protocol version][bincode payload]

The length field covers the protocol version + payload bytes.

Errors

Returns an error if serialization fails.