Send an outbound SMS (queued; delivered asynchronously)

Body for POST /v1/sms/send.

agent_id is required — the outbound-context helper enforces both agent-↔-number authorization and the org's destination-country allowlist. Conversation + session are resolved (or created) server-side from the from/to pair so callers stay stateless.

idempotency_key is optional but strongly recommended: when supplied, a repeat POST with the same key claims the same durable operation instead of dispatching a second send. Omit only for truly fire-once flows.

Response from POST /v1/sms/send.

Returns the routing context the caller (typically the shared brain) can use to correlate this send with the broader conversation state. Echoes the idempotency_key the caller supplied (or null) so clients can log it and retry safely with the same value.

operation_id is the durable SmsOutboundOperation.id (ENG-846) — a stable handle for downstream status/observability tooling (ENG-849) to poll delivery state without re-deriving it from conversation rows. A retried request with the same idempotency_key gets back the SAME operation_id.