Add a standalone codex-exec-server executable (#57589)

## What changed

Extract executor startup into `codex-exec-server-cli`, shared by `codex exec-server` and the new standalone binary. Preserve executor arguments, authentication, configuration overrides, and sandbox helper dispatch across both entry points.

Build the standalone binary with `cargo build -p codex-exec-server-cli --bin codex-exec-server` and run it with `codex-exec-server --listen stdio`.

## Testing

Add standalone CLI parsing and help coverage, plus an integration test for version reporting, stdio initialization, filesystem reads, process execution, and sandbox helper re-execution when supported.

GitOrigin-RevId: ab3a81c6b91b6f45e4d72d8add2e01af8372e97f

codex-rs/exec-server-cli/src/exec_server_telemetry_tests.rs

@@ -0,0 +1,33 @@
+#![cfg(test)]
+
+use std::time::Duration;
+
+use super::ShutdownBehavior;
+use super::run_until_shutdown_with_signals;
+
+#[tokio::test]
+async fn parent_disconnect_still_stops_executor_after_signal_listener_error() {
+    let (parent_sender, parent_receiver) = tokio::sync::oneshot::channel();
+    let (shutdown_sender, shutdown_receiver) = tokio::sync::oneshot::channel();
+    let run = async move {
+        let _ = shutdown_receiver.await;
+        Ok::<(), std::io::Error>(())
+    };
+    let task = tokio::spawn(run_until_shutdown_with_signals(
+        run,
+        async move {
+            let _ = parent_receiver.await;
+        },
+        std::future::ready(Err(std::io::Error::other("signal listener failed"))),
+        ShutdownBehavior::Graceful(shutdown_sender),
+    ));
+
+    tokio::task::yield_now().await;
+    parent_sender.send(()).expect("parent lifetime receiver");
+
+    tokio::time::timeout(Duration::from_secs(1), task)
+        .await
+        .expect("parent disconnect should stop the executor")
+        .expect("shutdown task should not panic")
+        .expect("executor should shut down successfully");
+}