fix(api): encode Realtime call offers and session configuration (#3736)

## Summary

Use generated request encoding for Realtime WebRTC setup so future
schema changes no longer require a parallel handwritten implementation.
Applications can send an SDP offer and optional session configuration
through the existing synchronous and asynchronous `calls.create`
methods.

Requests now preserve the API’s declared SDP and JSON content types, so
offers and session configuration reach the API in the expected format.
SDP-only requests use the supported raw-SDP form; the method signatures
and SDP-response interface stay the same.

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