Function write_message 

pub async fn write_message(
    writer: &mut (impl AsyncWriteExt + Unpin),
    msg: &Value,
) -> Result<(), NativeMessageError>

Write one native messaging frame to stdout.

Format: 4-byte little-endian length prefix, then UTF-8 JSON. Maximum output size is 64 MB per Chrome’s spec (host → extension direction).

Errors

Returns an error if the serialized message exceeds 64 MB or if the write fails.