Send 180 Ringing on a ringing inbound SIP leg (asynchronous)

Queues a SIP 180 Ringing provisional response with no SDP. Use when SIP_AUTO_RINGING=false (the default) and you want to indicate alerting before deciding to early-media or answer. Idempotent: each call emits another 180 — receivers tolerate re-sends. The HTTP call returns 202 as soon as the request is validated; SIP-level send failures surface as leg.command_failed with command="ring".