Removes a single ESA channel's association with a threshold alert

Use this endpoint to detach one channel from an alert while leaving the alert and its other channel associations untouched. To delete the entire alert, including all of its channel associations, use DELETE /esaalert/{esaAlertId} instead. Unlike POST /esaalert/{esaAlertId}/channel, the alert does not need to be active - detaching is non-destructive and often desired cleanup for a disabled alert. Auto-managed sentinel alerts cannot have their channel removed this way, since a sentinel alert exists because of its one channel. Returns 404, not 400, if the channel isn’t currently associated with the alert, so a repeated request reads as “already in the desired state” rather than a malformed request. Also returns 404 if the channel exists but is outside the caller’s accessible scope, rather than exposing that the channel belongs elsewhere. Historical EsaAlertEvent rows for this alert/channel pairing are preserved, not deleted, so alert history remains available after removal.