Function open_download 

pub async fn open_download<Source: DavDownloadSource>(
    source: &Source,
    path: &DavPath,
    body: DavDownloadBody,
) -> Result<Option<DavOpenedDownload>, DavDownloadOpenError>

Opens the storage stream selected by a download plan and verifies its declared length.

Full, single-range, and every final multipart segment carry their exact selected length. Multipart segments are all opened and checked before the returned stream can emit bytes; empty plans never call the backend.

Errors

Returns DavDownloadOpenError when a backend open fails or its length is inconsistent.