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/config/src/config_toml.rs

@@ -448,9 +448,9 @@ pub enum RealtimeWsMode {
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, Default, PartialEq, Eq, JsonSchema)]
 #[serde(rename_all = "snake_case")]
 pub enum RealtimeTransport {
+    #[default]
     #[serde(rename = "webrtc")]
     WebRtc,
-    #[default]
     Websocket,
 }