Support ChatGPT realtime calls auth

Route realtime call auth through CoreAuthProvider and send JSON payloads for non-v1 realtime calls while preserving the v1 multipart request shape.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/endpoint/realtime_websocket/methods_v2.rs

@@ -126,3 +126,7 @@ pub(super) fn session_update_session(
         },
     }
 }
+
+pub(super) fn webrtc_intent() -> Option<&'static str> {
+    None
+}