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/lib.rs

@@ -0,0 +1,35 @@
+//! Startup shared by the full Codex CLI and the standalone executor.
+//! Internal sandbox and filesystem re-exec modes always dispatch before argument parsing.
+
+use clap::CommandFactory;
+use clap::Parser;
+use codex_utils_cli::CliConfigOverrides;
+use codex_utils_cli::collect_config_overrides;
+
+mod command;
+mod config;
+mod exec_server_auth;
+mod exec_server_credential;
+mod exec_server_telemetry;
+
+pub use command::ExecServerCommand;
+
+#[derive(Debug, Parser)]
+#[command(name = "codex-exec-server", bin_name = "codex-exec-server", version)]
+struct ExecServerCli {
+    #[command(flatten)]
+    config_overrides: CliConfigOverrides,
+    #[command(flatten)]
+    command: ExecServerCommand,
+}
+
+/// Starts the executor after dispatching internal helper modes and preparing runtime paths.
+pub fn run_standalone() -> anyhow::Result<()> {
+    codex_arg0::arg0_dispatch_or_else(|paths| async move {
+        let args: Vec<_> = std::env::args_os().collect();
+        let mut cli = ExecServerCli::parse_from(&args);
+        cli.config_overrides = collect_config_overrides(ExecServerCli::command(), args)
+            .unwrap_or_else(|error| error.exit());
+        cli.command.run(&paths, &cli.config_overrides).await
+    })
+}