Cancel a session

Requests cancellation of a running session. The optional body selects the cancel mode: graceful (default, also used when the body is absent) stops the agent and tries to preserve the workspace with a final snapshot; hard stops immediately and relies on the last verified checkpoint. Sending hard while a graceful cancel is in progress upgrades it. Either way an explicitly cancelled Session ends suspended, never error. Returns whether the request newly flagged (or upgraded) the cancellation. Cancellation is asynchronous: requested: true means the stop is under way, and status is the Session's status after this request, so it is suspended when the Session had not started and still running while a running Session stops. Poll the Session until its status leaves running.

_{Scope: run}