Function ensure_lock_target_exists 

pub async fn ensure_lock_target_exists(
    filesystem: &dyn DavFileSystem,
    path: &DavPath,
) -> Result<bool, FsError>

Ensures that a LOCK target exists, creating an empty lock-null file when allowed.

Existing resources are left untouched. A missing collection target remains missing because creating its hierarchy is outside LOCK semantics.