DropboxTarget: path-addressed like OneDrive/WebDAV; create_folder_v2 creates
the whole intermediate path in one call; upload-session chunking above
LARGE_FILE_THRESHOLD (start/append_v2/finish).
BoxTarget: Box addresses items by numeric ID, not path, so ensure_dir/exists/
put all walk (and lazily create) the folder chain from root ("0") by listing
each level's children. Always uploads via Box's session API regardless of
file size rather than its simple multipart endpoint, since the session API
takes plain PUT bodies (consistent with every other target here) and needs
no new reqwest feature; Box computes and returns each part's digest, so no
local hashing is needed either.
Both follow the same documented ceiling as OneDriveTarget's put_chunked
(M4.1): the upload-session id lives only for one put() call, not persisted
across a durable-queue retry after a restart.
Tests: in-process std::net mock servers for both providers (same pattern as
the existing MockNextcloud/MockGraph), each exercising a real multi-chunk
upload end-to-end.
iamdoubz
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