fix: preserve zsh-fork escalation fds in unified-exec PTYs

[bolinfest]

Why

zsh-fork sessions routed through unified-exec need fd-safe process launch semantics so escalation works reliably for long-lived PTY sessions.

This commit fixes fd handling at each handoff boundary, including a subtle spawn-error path where aggressive fd pruning could hide exec failures.

What Changed

• Fixed wrapper fd transfer in shell-escalation (shell-escalation/src/unix/escalate_client.rs):
  • duplicate stdin/stdout/stderr before sending fds to the server
  • send destination stdio numbers (0/1/2) in SuperExecMessage while transferring the duplicated fds
• Added inherited-fd plumbing for unified-exec process startup:
  • SpawnLifecycle::inherited_fds() in core/src/unified_exec/process.rs
  • process manager passes inherited fds into PTY spawn (core/src/unified_exec/process_manager.rs)
  • zsh-fork backend exposes the escalation socket fd via the lifecycle (core/src/tools/runtimes/shell/zsh_fork_backend.rs)
• Added PTY spawn support for explicitly preserved inherited fds in utils/pty/src/pty.rs via spawn_process_with_inherited_fds(...).
• Fixed Unix pre-exec fd pruning in utils/pty/src/pty.rs to not close FD_CLOEXEC descriptors, preserving Rust's internal exec-error pipe so missing executables still fail at spawn().
• Added PTY regressions in utils/pty/src/tests.rs for:
  • preserved-fd spawn behavior
  • preserved-fd Python REPL continuity
  • missing executable propagation when inherited-fd spawn path is used

Verification

• cargo clippy -p codex-utils-pty --tests -- -D warnings
• cargo test -p codex-utils-pty
• cargo test -p codex-shell-escalation

---

Stack created with Sapling. Best reviewed with ReviewStack.

• refactor: route zsh-fork through unified exec #13432
• -> fix: preserve zsh-fork escalation fds in unified-exec PTYs #13644

[chatgpt-codex-connector]

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[jif-oai]

Will require some heavy testing in the end but I guess it's in the follow up

Approving to unblock but please make sure the point raised (mainly for non-pty) are not real issues