Starts a new call for the Web channel. Requires agentId, callId, sdp.
Optional fields: businessNumber, contactName, from, language, accent, modelProvider.

On HTTP 200, the response always includes sdpAnswer, connectionId, callId,
relayMode, and testMode. Use connectionId for debugging; correlate lifecycle
polling with the client-supplied callId via GET /voice/calls/{callId}/status.

Error bodies are usually JSON {code, message} for 400/403; some 500 paths may
return plain text (e.g. SDP processing failure).