Function write_sync_message_json 

pub fn write_sync_message_json(
    writer: &mut impl Write,
    message: &SyncMessage,
) -> Result<()>

Writes the compact JSON representation used by SyncMessage serialization.

Unlike serde_json::to_vec, this function does not require a second payload-sized allocation. It preserves the derived Serde field order and escaping so persistent providers can stream directly to bounded storage.