Function enforce_if_header_with_backends 

pub async fn enforce_if_header_with_backends(
    if_header: Option<&IfHeader>,
    filesystem: &dyn DavFileSystem,
    lock_system: &dyn DavLockSystem,
    request_path: &DavPath,
    prefix: &str,
    request_scheme: &str,
    request_host: &str,
) -> Result<(), DavIfEvaluationError>

Resolves If state from the canonical filesystem and lock backend ports.

Products normally use this entrypoint instead of implementing a transport-local resolver. A missing resource contributes no ETag, while all other filesystem failures preserve the backend error boundary.