Prewarm code-mode host connections at session startup (#40678)

## What changed

- Add the opt-in `code_mode_prewarm` feature to establish the code-mode host session during startup, before the first turn.
- Make in-progress host initialization cancellable so a stalled prewarm does not block shutdown.
- Share a failed connection attempt with concurrent callers instead of immediately starting another attempt.

## Testing

- Verify app-server contacts the configured host before the first turn and can shut down while that connection is stalled.
- Exercise shared remote-host behavior with prewarming enabled.

GitOrigin-RevId: 3cfde5509be7b4aba80112c36c5616b9a9b632c6

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

@@ -34,6 +34,35 @@ async fn app_server_shares_flag_selected_grpc_code_mode_host_across_threads() ->
     assert_shared_remote_code_mode_host("grpc://127.0.0.1:0").await
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn app_server_prewarms_flag_selected_code_mode_host_before_first_turn() -> Result<()> {
+    let model_server = responses::start_mock_server().await;
+    let codex_home = TempDir::new()?;
+    MockResponsesConfig::new(&model_server.uri())
+        .enable_feature(Feature::CodeModePrewarm)
+        .write(codex_home.path())?;
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await?;
+    let host_url = format!("ws://{}", listener.local_addr()?);
+    let mut app_server = TestAppServer::builder()
+        .with_codex_home(codex_home.path())
+        .with_args(&["--code-mode-host", &host_url])
+        .build_initialized_with_timeout(DEFAULT_READ_TIMEOUT)
+        .await?;
+    app_server
+        .start_thread(ThreadStartParams::default())
+        .await?;
+
+    let (_stalled_connection, _) = timeout(DEFAULT_READ_TIMEOUT, listener.accept())
+        .await
+        .context("code-mode host was not contacted before the first turn")??;
+    let status = timeout(Duration::from_secs(5), app_server.shutdown_gracefully())
+        .await
+        .context("stalled code-mode prewarm blocked thread shutdown")??;
+    assert!(status.success(), "app-server did not exit successfully");
+    Ok(())
+}
+
 async fn assert_shared_remote_code_mode_host(listen_url: &str) -> Result<()> {
     let host_program = codex_utils_cargo_bin::cargo_bin("codex-code-mode-host")?;
     let mut code_mode_host = Command::new(host_program)
@@ -91,6 +120,7 @@ async fn assert_shared_remote_code_mode_host(listen_url: &str) -> Result<()> {
     let codex_home = TempDir::new()?;
     MockResponsesConfig::new(&model_server.uri())
         .enable_feature(Feature::CodeModeOnly)
+        .enable_feature(Feature::CodeModePrewarm)
         .write(codex_home.path())?;
     let original_config = std::fs::read_to_string(codex_home.path().join("config.toml"))?;
     let mut app_server = TestAppServer::builder()