Update realtime websocket API (#13265)

- migrate the realtime websocket transport to the new session and
handoff flow
- make the realtime model configurable in config.toml and use API-key
auth for the websocket

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

codex-rs/core/tests/common/test_codex.rs

@@ -126,6 +126,7 @@ impl TestCodexBuilder {
         let base_url_clone = base_url.clone();
         self.config_mutators.push(Box::new(move |config| {
             config.model_provider.base_url = Some(base_url_clone);
+            config.experimental_realtime_ws_model = Some("realtime-test-model".to_string());
             config.features.enable(Feature::ResponsesWebsockets);
         }));
         self.build_with_home_and_base_url(base_url, home, None)