Add expected JSON snippets to examples/hello-voice

## Task

`examples/hello-voice/README.md` walks register → sample call with curl, but does not show a trimmed successful response. Newcomers cannot tell if they succeeded.

## Scope

- Add abbreviated example JSON for register (tokens present, no secrets in full) and `run-sample-call` (session id, transcript or status)
- Note mock providers and `DEMO_ENABLED`
- Keep the example runnable without LiveKit

## Acceptance criteria

- [ ] Snippets match current API field names (verify against `/api/v1/docs` or tests)
- [ ] No real credentials
- [ ] LF only

Start with examples/hello-voice/README.md, then verify the current response fields against /api/v1/docs or the relevant tests. Add trimmed register and run-sample-call JSON without real credentials, document mock providers and DEMO_ENABLED, and confirm the example remains runnable without LiveKit and uses LF line endings.

Written by the indexing model from the issue text.