Place a test voice call via Sierra

Dry-run/confirm-gated endpoint to place a Sierra voice-agent test call.

Requires scope: voice_calls:write

Two possible 202 response shapes, depending on payload.confirmation_token:

• Dry run (confirmation_token omitted)::

  { "mode": "dry_run", "confirmation_token": "<token>", "validated_config": { "purpose": ..., "target": {...}, "inputs": {...}, "resolved_agent": ..., "caller_id_number": ... } }

• Confirmed (confirmation_token supplied and valid)::

  {"mode": "confirmed", "call_id": "<sierra call id>", "status": "dispatched"}