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/main.rs

@@ -0,0 +1,14 @@
+//! Standalone executor entry point; startup and sandbox dispatch are shared with Codex.
+
+#[cfg(all(
+    target_os = "linux",
+    target_env = "musl",
+    any(target_arch = "x86_64", target_arch = "aarch64")
+))]
+#[global_allocator]
+static ALLOCATOR: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
+
+fn main() -> anyhow::Result<()> {
+    codex_build_info::initialize!();
+    codex_exec_server_cli::run_standalone()
+}