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,
the Celery task id is derived from it so repeat POSTs with the same key
dedup at the broker. Omit only for truly fire-once flows.