Function encode_trailer_block 

pub fn encode_trailer_block(trailer: &Trailer) -> Bytes

Encode just the trailing [u32 len | Trailer bytes] block. Used by the streaming response path, which forwards the message block straight from the inner gRPC frame (its [u32 len | message] layout already matches ours once the 1-byte compression flag is dropped) and only needs to append this at the end.