The cloud backend previously polled tree_resp["retrieval_ready"]
as the ready signal. Empirically this flag is not a reliable
indicator — docs can reach status=="completed" without
retrieval_ready flipping, causing col.add() to wait until the 10
min timeout before giving up on otherwise-successful uploads.
The cloud API's canonical ready signal is status=="completed";
switch the poll to check that instead.
162b70d825