Function run_copy_prefix 

pub fn run_copy_prefix(
    client: &mut RespClient,
    src_prefix: &[u8],
    dst_prefix: &[u8],
    rate: u64,
) -> Result<u64>

copy-prefix: SCAN src prefix, re-key under dst prefix via read+rebuild frames (the server has no COPY verb; TTL carried as absolute PEXPIREAT). Rate-limited per source key.