Launch Linux pipe processes through a fresh setup helper (#47612)

## Why

Linux pipe launches need session, parent-death signal, and descriptor setup. Move this work into a fresh, single-threaded executable image to avoid forking the app-server for child setup.

## What changed

- Register an early setup helper and launch it through `posix_spawn` for Linux pipe processes.
- Start the helper with an empty environment, then transfer the target environment over a control socket so loader settings cannot interfere with helper startup.
- Await target execution asynchronously, propagate setup and execution errors, and kill incomplete launches on cancellation.
- Preserve direct spawning as a fallback when the helper is unavailable or fails before target execution.

## Testing

Add coverage for launch semantics, avoiding fork, loader environment isolation, early argument dispatch, closed standard descriptors, and fallback under bootstrap failure or descriptor pressure. Add an app-server regression test that reuses a process handle after an execution failure.

GitOrigin-RevId: 079c673f0f22ebd531f8c57e5338095cc98acfdc

codex-rs/exec-server/src/local_process.rs

@@ -1251,6 +1251,20 @@ mod tests {
     #[cfg(not(target_os = "windows"))]
     use crate::protocol::NetworkPolicyRequestResponse;
 
+    #[cfg(target_os = "linux")]
+    #[ctor::ctor]
+    fn initialize_spawn_helper() {
+        use std::os::unix::ffi::OsStringExt;
+        let command_line = std::fs::read("/proc/self/cmdline").expect("test command line");
+        codex_utils_pty::init_spawn_helper(
+            command_line
+                .strip_suffix(&[0])
+                .unwrap_or(&command_line)
+                .split(|byte| *byte == 0)
+                .map(|arg| std::ffi::OsString::from_vec(arg.to_vec())),
+        );
+    }
+
     fn test_exec_params(env: HashMap<String, String>) -> ExecParams {
         ExecParams {
             metadata: None,