fix: preserve zsh-fork escalation fds across unified-exec spawn paths (#13644)

## Why

`zsh-fork` sessions launched through unified-exec need the escalation
socket to survive the wrapper -> server -> child handoff so later
intercepted `exec()` calls can still reach the escalation server.

The inherited-fd spawn path also needs to avoid closing Rust's internal
exec-error pipe, and the shell-escalation handoff needs to tolerate the
receive-side case where a transferred fd is installed into the same
stdio slot it will be mapped onto.

## What Changed

- Added `SpawnLifecycle::inherited_fds()` in
`codex-rs/core/src/unified_exec/process.rs` and threaded inherited fds
through `codex-rs/core/src/unified_exec/process_manager.rs` so
unified-exec can preserve required descriptors across both PTY and
no-stdin pipe spawn paths.
- Updated `codex-rs/core/src/tools/runtimes/shell/zsh_fork_backend.rs`
to expose the escalation socket fd through the spawn lifecycle.
- Added inherited-fd-aware spawn helpers in
`codex-rs/utils/pty/src/pty.rs` and `codex-rs/utils/pty/src/pipe.rs`,
including Unix pre-exec fd pruning that preserves requested inherited
fds while leaving `FD_CLOEXEC` descriptors alone. The pruning helper is
now named `close_inherited_fds_except()` to better describe that
behavior.
- Updated `codex-rs/shell-escalation/src/unix/escalate_client.rs` to
duplicate local stdio before transfer and send destination stdio numbers
in `SuperExecMessage`, so the wrapper keeps using its own
`stdin`/`stdout`/`stderr` until the escalated child takes over.
- Updated `codex-rs/shell-escalation/src/unix/escalate_server.rs` so the
server accepts the overlap case where a received fd reuses the same
stdio descriptor number that the child setup will target with `dup2`.
- Added comments around the PTY stdio wiring and the overlap regression
helper to make the fd handoff and controlling-terminal setup easier to
follow.

## Verification

- `cargo test -p codex-utils-pty`
- covers preserved-fd PTY spawn behavior, PTY resize, Python REPL
continuity, exec-failure reporting, and the no-stdin pipe path
- `cargo test -p codex-shell-escalation`
- covers duplicated-fd transfer on the client side and verifies the
overlap case by passing a pipe-backed stdin payload through the
server-side `dup2` path

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/13644).
* #14624
* __->__ #13644

codex-rs/utils/pty/src/pipe.rs

@@ -102,11 +102,15 @@ async fn spawn_process_with_stdin_mode(
     env: &HashMap<String, String>,
     arg0: &Option<String>,
     stdin_mode: PipeStdinMode,
+    inherited_fds: &[i32],
 ) -> Result<SpawnedProcess> {
     if program.is_empty() {
         anyhow::bail!("missing program for pipe spawn");
     }
 
+    #[cfg(not(unix))]
+    let _ = inherited_fds;
+
     let mut command = Command::new(program);
     #[cfg(unix)]
     if let Some(arg0) = arg0 {
@@ -115,11 +119,14 @@ async fn spawn_process_with_stdin_mode(
     #[cfg(target_os = "linux")]
     let parent_pid = unsafe { libc::getpid() };
     #[cfg(unix)]
+    let inherited_fds = inherited_fds.to_vec();
+    #[cfg(unix)]
     unsafe {
         command.pre_exec(move || {
             crate::process_group::detach_from_tty()?;
             #[cfg(target_os = "linux")]
             crate::process_group::set_parent_death_signal(parent_pid)?;
+            crate::pty::close_inherited_fds_except(&inherited_fds);
             Ok(())
         });
     }
@@ -250,7 +257,7 @@ pub async fn spawn_process(
     env: &HashMap<String, String>,
     arg0: &Option<String>,
 ) -> Result<SpawnedProcess> {
-    spawn_process_with_stdin_mode(program, args, cwd, env, arg0, PipeStdinMode::Piped).await
+    spawn_process_with_stdin_mode(program, args, cwd, env, arg0, PipeStdinMode::Piped, &[]).await
 }
 
 /// Spawn a process using regular pipes, but close stdin immediately.
@@ -261,5 +268,27 @@ pub async fn spawn_process_no_stdin(
     env: &HashMap<String, String>,
     arg0: &Option<String>,
 ) -> Result<SpawnedProcess> {
-    spawn_process_with_stdin_mode(program, args, cwd, env, arg0, PipeStdinMode::Null).await
+    spawn_process_no_stdin_with_inherited_fds(program, args, cwd, env, arg0, &[]).await
+}
+
+/// Spawn a process using regular pipes, close stdin immediately, and preserve
+/// selected inherited file descriptors across exec on Unix.
+pub async fn spawn_process_no_stdin_with_inherited_fds(
+    program: &str,
+    args: &[String],
+    cwd: &Path,
+    env: &HashMap<String, String>,
+    arg0: &Option<String>,
+    inherited_fds: &[i32],
+) -> Result<SpawnedProcess> {
+    spawn_process_with_stdin_mode(
+        program,
+        args,
+        cwd,
+        env,
+        arg0,
+        PipeStdinMode::Null,
+        inherited_fds,
+    )
+    .await
 }