feat(api): support backend-mediated Realtime WebRTC calls (#2469)

## Summary

Add `client.realtime.calls.create` so server-side applications can
exchange an SDP offer and optional session configuration for an SDP
answer.

This makes WebRTC signaling available through the SDK instead of
requiring a handwritten HTTP request. The request preserves the API’s
SDP and JSON content types, and the returned `Response` exposes the SDP
answer for the browser.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>