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/exec-server/testing/exec_server.rs

@@ -1,12 +1,28 @@
 //! Minimal exec-server fixture for Bazel-only integration tests.
 //!
 //! Linking only exec-server avoids depending on the full Codex CLI binary
-//! when a test only needs a WebSocket executor endpoint.
+//! when a test only needs a WebSocket executor endpoint. It handles the arg0
+//! helper mode because sandboxed process requests re-exec this binary.
 
 use codex_exec_server::ExecServerRuntimePaths;
+#[cfg(unix)]
+use std::ffi::OsStr;
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    #[cfg(unix)]
+    {
+        let mut args = std::env::args_os();
+        let _ = args.next();
+        if args.next().as_deref()
+            == Some(OsStr::new(
+                codex_exec_server::CODEX_ARG0_EXEC_HELPER_ARG1,
+            ))
+        {
+            codex_exec_server::run_arg0_exec_helper_main();
+        }
+    }
+
     let current_exe = std::env::current_exe()?;
     let runtime_paths =
         ExecServerRuntimePaths::new(current_exe, /*codex_linux_sandbox_exe*/ None)?;