Complete multipart upload

Assemble the uploaded parts into the final object and add it to your media library. Send every part with the ETag returned by its PUT. A 500 is always retryable — whatever we have (your parts, or the assembled object) is kept, so retry this call rather than re-uploading. A 400 is final: either the part list was wrong, or the file itself was rejected (empty, wrong type, too large, no readable preview) and its bytes have been deleted. A 403 means quota or permissions and is raised before assembly, so your parts are still open — call abort yourself. Calling this again after it already succeeded returns the same upload instead of charging you twice.