Add app-server v2 thread realtime API (#12715)

Add experimental `thread/realtime/*` v2 requests and notifications, then
route app-server realtime events through that thread-scoped surface with
integration coverage.

---------

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

codex-rs/app-server/tests/suite/v2/experimental_api.rs

@@ -10,6 +10,7 @@ use codex_app_server_protocol::JSONRPCMessage;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::MockExperimentalMethodParams;
 use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ThreadRealtimeStartParams;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadStartResponse;
 use pretty_assertions::assert_eq;
@@ -50,6 +51,40 @@ async fn mock_experimental_method_requires_experimental_api_capability() -> Resu
     Ok(())
 }
 
+#[tokio::test]
+async fn realtime_conversation_start_requires_experimental_api_capability() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+
+    let init = mcp
+        .initialize_with_capabilities(
+            default_client_info(),
+            Some(InitializeCapabilities {
+                experimental_api: false,
+                opt_out_notification_methods: None,
+            }),
+        )
+        .await?;
+    let JSONRPCMessage::Response(_) = init else {
+        anyhow::bail!("expected initialize response, got {init:?}");
+    };
+
+    let request_id = mcp
+        .send_thread_realtime_start_request(ThreadRealtimeStartParams {
+            thread_id: "thr_123".to_string(),
+            prompt: "hello".to_string(),
+            session_id: None,
+        })
+        .await?;
+    let error = timeout(
+        DEFAULT_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+    assert_experimental_capability_error(error, "thread/realtime/start");
+    Ok(())
+}
+
 #[tokio::test]
 async fn thread_start_mock_field_requires_experimental_api_capability() -> Result<()> {
     let server = create_mock_responses_server_sequence_unchecked(Vec::new()).await;