Preserve custom arg0 for sandboxed exec-server processes (#34497)

## Why

Sandbox wrappers replaced the process launch command and did not carry an
`ExecParams.arg0` override through to the inner process.

## What changed

- Route sandboxed Unix launches with a custom `arg0` through a helper mode that
  re-execs the requested program with the override.
- Expose the helper executable to the filesystem sandbox and dispatch its mode
  from Codex and exec-server test binaries.

## Testing

Add coverage for the prepared sandbox command and an end-to-end remote process
that verifies both the custom `arg0` and filesystem restrictions.

GitOrigin-RevId: c9f8eef3906d184e670184c2eeed250d5895a9ca

codex-rs/app-server/src/bin/exec_server.rs

@@ -2,8 +2,8 @@
 //!
 //! This mirrors `//codex-rs/exec-server/testing:exec-server` so Cargo-backed
 //! app-server integration tests can receive `CARGO_BIN_EXE_exec-server`. It
-//! also handles the filesystem-helper argv mode because exec-server re-execs
-//! `codex_self_exe` for sandboxed filesystem requests.
+//! also handles the helper argv modes because exec-server re-execs
+//! `codex_self_exe` for sandboxed filesystem and process requests.
 
 use codex_exec_server::ExecServerRuntimePaths;
 use std::ffi::OsStr;
@@ -13,7 +13,12 @@ const CODEX_LINUX_SANDBOX_EXE_ENV_VAR: &str = "CODEX_TEST_LINUX_SANDBOX_EXE";
 fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     let mut args = std::env::args_os();
     let _ = args.next();
-    if args.next().as_deref() == Some(OsStr::new(codex_exec_server::CODEX_FS_HELPER_ARG1)) {
+    let argv1 = args.next();
+    #[cfg(unix)]
+    if argv1.as_deref() == Some(OsStr::new(codex_exec_server::CODEX_ARG0_EXEC_HELPER_ARG1)) {
+        codex_exec_server::run_arg0_exec_helper_main();
+    }
+    if argv1.as_deref() == Some(OsStr::new(codex_exec_server::CODEX_FS_HELPER_ARG1)) {
         codex_exec_server::run_fs_helper_main();
     }