fix: preserve zsh-fork escalation fds across unified-exec spawn paths

[bolinfest]

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

---

Stack created with Sapling. Best reviewed with ReviewStack.

• refactor: route zsh-fork through unified exec #14624
• -> fix: preserve zsh-fork escalation fds across unified-exec spawn paths #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