Address exec-server sandbox review comments

- preserve sandbox child environment variables from shared launch requests
- dispatch codex-exec-server through codex-arg0 so helper arg0 is executable
- send sandbox preference to remote servers and report the server-selected sandbox

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/tests/process.rs

@@ -75,6 +75,7 @@ async fn exec_server_starts_process_over_websocket() -> anyhow::Result<()> {
         process_start_response,
         ExecResponse {
             process_id: ProcessId::from("proc-1"),
+            sandbox: codex_sandboxing::SandboxType::None,
         }
     );