make webrtc the default experience (#17188)

## Summary
- make realtime default to the v2 WebRTC path
- keep partial realtime config tables inheriting
`RealtimeConfig::default()`

## Validation
- CI found a stale config-test expectation; fixed in 974ba51bb3
- just fmt
- git diff --check

---------

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

codex-rs/core/src/config/config_tests.rs

@@ -6498,10 +6498,8 @@ voice = "marin"
     assert_eq!(
         config.realtime,
         RealtimeConfig {
-            version: RealtimeWsVersion::V2,
-            session_type: RealtimeWsMode::Conversational,
-            transport: RealtimeTransport::Websocket,
             voice: Some(RealtimeVoice::Marin),
+            ..RealtimeConfig::default()
         }
     );
     Ok(())