Legacy partial canvas graph extension

Account-scoped key (1 to 255 characters, ideally a UUID). Retries must preserve the credential workspace, effective API version, URL, content type and exact body bytes, including multipart boundaries; different inputs return 409 idempotency_key_reused. Completed responses, including 500 errors, replay for 24 hours after completion with current resource access checked again. Running or unconfirmed executions return 409 idempotency_request_in_progress and do not expire automatically. A receipt persistence failure returns 503 idempotency_persistence_failed: retry only with the same key and inspect the operation outcome before starting a new request.