Add WebRTC transport to realtime start (#16960)

Adds WebRTC startup to the experimental app-server
`thread/realtime/start` method with an optional transport enum. The
websocket path remains the default; WebRTC offers create the realtime
session through the shared start flow and emit the answer SDP via
`thread/realtime/sdp`.

---------

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

codex-rs/codex-api/src/endpoint/memories.rs

@@ -72,6 +72,7 @@ mod tests {
     use crate::provider::RetryConfig;
     use async_trait::async_trait;
     use codex_client::Request;
+    use codex_client::RequestBody;
     use codex_client::Response;
     use codex_client::StreamResponse;
     use codex_client::TransportError;
@@ -213,7 +214,11 @@ mod tests {
             request.url,
             "https://example.com/api/codex/memories/trace_summarize"
         );
-        let body = request.body.expect("request body should be present");
+        let body = request
+            .body
+            .as_ref()
+            .and_then(RequestBody::json)
+            .expect("request body should be JSON");
         assert_eq!(body["model"], "gpt-test");
         assert_eq!(body["traces"][0]["id"], "trace-1");
         assert_eq!(