Cancel Run

Cancel an in-flight run.

Idempotent: a run already in a terminal state (completed / failed / cancelled) is returned as-is. Once cancelled, the gateway refuses every subsequent chat_completions call from the worker — so the project cannot be billed further regardless of whether the worker subprocess notices the cancel itself.

The worker keeps running for at most one more bcode step (consuming no LLM tokens because CP 409s them) and then exits when bcode realizes the next tool call has no model behind it.