Default realtime startup to v2 model (#17183)

- Default realtime sessions to v2 and gpt-realtime-1.5 when no override
is configured.
- Add Op::RealtimeConversationStart integration coverage and keep
v1-specific tests explicit.

---------

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

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

@@ -32,6 +32,7 @@ use codex_protocol::openai_models::ModelsResponse;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
+use codex_protocol::protocol::RealtimeConversationVersion as RealtimeWsVersion;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::SessionConfiguredEvent;
 use codex_protocol::protocol::SessionSource;
@@ -453,6 +454,7 @@ impl TestCodexBuilder {
             config.model_provider.base_url = Some(base_url_clone);
             config.model_provider.supports_websockets = true;
             config.experimental_realtime_ws_model = Some("realtime-test-model".to_string());
+            config.realtime.version = RealtimeWsVersion::V1;
         }));
         let test_env = TestEnv::local().await?;
         Box::pin(self.build_with_home_and_base_url(base_url, home, /*resume_from*/ None, test_env))