Cancel a queued call (path-param form)

REST-conventional path-param variant of [`POST /conversation/cancel`](#operation/cancelCallByBody). **Behavior differs from the body form.** This path-param endpoint only cancels calls that are still in the `IN_QUEUE` state — calls that have already started dialing or are in progress return `400 Bad Request` with `errors: ["Conversation with ID ... is not in queue and cannot be cancelled"]`. Use the body form (`POST /conversation/cancel`) if you need to cancel an in-progress call. The path param is the `callId` string (e.g. `CALL-1778226705739-7e4c17`), **not** the Mongo `_id`. Passing `_id` returns `404 No conversation found`.