Cooperatively stop a POST /conversations/stream or POST /conversations/{conversationId}/messages/stream run that is still generating, using the runId sent when that stream started.

This is a synchronous JSON ack, not another SSE stream. The cancelled run's own stream (if still connected) receives a terminal frame with a stopped status and whatever partial answer had already generated; nothing further is delivered here.

A runId that already finished or was never registered is not an error either, but it is answered { cancelled: true }, not false: the query service's KV-backed run registry publishes the cancel for other workers and acknowledges it as cancelled (a known defect). { cancelled: false } is only returned when that publish fails.

Only the conversation's owner (matched on userId) can cancel; any other caller, or a missing, deleted or agent conversation, is 404.

runId must belong to a run started on THIS conversationId — a runId that exists but was registered under a different conversation (even one owned by the same caller) is rejected with 403, same as a runId owned by a different user/org.