Ensure Workspace Box

Bring up the box a command must be pinned to, and name it.

A caller that names a notebook gets that notebook's box; one that names none gets the workspace's own. The gateway plane has no box a workspace owns -- boxes come up only as a side effect of a notebook's kernel start -- so a command that names no notebook has nothing to run on (QUA-3067), and a clone meets this first: the notebook it would have to open for a box is a file at the root it requires to be empty.

Answers 202-style with an operation_id the caller polls at GET /operations/{id}, exactly as /kernels/start does. Provisioning is a machine plus the gateway's own 240s health wait, and a request held open with no bytes flowing for that long is what an ALB idle timeout ends.