Create a widget voice session

Mints short-lived credentials for a browser voice conversation with the widget's agent: a signed WebSocket URL, and when available a WebRTC conversation token (preferred, for echo cancellation and noise removal; fall back to `signed_url` if WebRTC cannot connect). Only for widgets with `voice_enabled: true`. No request body; call it per click and do not cache the result. Credentials, origin check and limits are the same as `POST /widget/init`. It does not need a session. **Side effects.** Calls the voice engine to mint the credentials for the agent's voice profile. Minting records no usage by itself; the voice conversation that follows runs on the widget's agent. **Idempotency.** Each call mints fresh credentials; retrying is harmless, and unused URLs expire. **Access** - **Required scope:** None; `jl_` API keys are not used. The widget credential plus a matching `x-widget-parent-origin` authorise the call. - **Rate limit:** Widget public — 60 requests/min per IP, plus the widget's `rate_limit_rpm` per credential and 300 requests/min per workspace; the general limiter also counts 120 requests/min per IP. See [Rate limits](/rate-limits). - **Plan:** Available on every plan.