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

Same synchronous JSON ack contract as the assistant POST /conversations/{conversationId}/cancel — both forward to the same backend cancellation endpoint, since the run registry is keyed by runId alone.

cancelled reports whether a stop request was issued, not whether a run was actually stopped. With the default shared (key-value backed) run registry, a runId that the answering worker does not hold is published for whichever worker owns it, and the answer is { cancelled: true } even when the run already finished or was never registered. { cancelled: false } is returned for such a runId only when the registry is in-process, or when the stop request could not be published.

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.