refactor: prepare unified exec for zsh-fork backend (#13392)

## Why

`shell_zsh_fork` already provides stronger guarantees around which
executables receive elevated permissions. To reuse that machinery from
unified exec without pushing Unix-specific escalation details through
generic runtime code, the escalation bootstrap and session lifetime
handling need a cleaner boundary.

That boundary also needs to be safe for long-lived sessions: when an
intercepted shell session is closed or pruned, any in-flight approval
workers and any already-approved escalated child they spawned must be
torn down with the session, and the inherited escalation socket must not
leak into unrelated subprocesses.

## What Changed

- Extracted a reusable `EscalationSession` and
`EscalateServer::start_session(...)` in `shell-escalation` so callers
can get the wrapper/socket env overlay and keep the escalation server
alive without immediately running a one-shot command.
- Documented that `EscalationSession::env()` and
`ShellCommandExecutor::run(...)` exchange only that env overlay, which
callers must merge into their own base shell environment.
- Clarified the prepared-exec helper boundary in `core` by naming the
new helper APIs around `ExecRequest`, while keeping the legacy
`execute_env(...)` entrypoints as thin compatibility wrappers for
existing callers that still use the older naming.
- Added a small post-spawn hook on the prepared execution path so the
parent copy of the inheritable escalation socket is closed immediately
after both the existing one-shot shell-command spawn and the
unified-exec spawn.
- Made session teardown explicit with session-scoped cancellation:
dropping an `EscalationSession` or canceling its parent request now
stops intercept workers, and the server-spawned escalated child uses
`kill_on_drop(true)` so teardown cannot orphan an already-approved
child.
- Added `UnifiedExecBackendConfig` plumbing through `ToolsConfig`, a
`shell::zsh_fork_backend` facade, and an opaque unified-exec
spawn-lifecycle hook so unified exec can prepare a wrapped `zsh -c/-lc`
request without storing `EscalationSession` directly in generic
process/runtime code.
- Kept the existing `shell_command` zsh-fork behavior intact on top of
the new bootstrap path. Tool selection is unchanged in this PR: when
`shell_zsh_fork` is enabled, `ShellCommand` still wins over
`exec_command`.

## Verification

- `cargo test -p codex-shell-escalation`
  - includes coverage for `start_session_exposes_wrapper_env_overlay`
  - includes coverage for `exec_closes_parent_socket_after_shell_spawn`
- includes coverage for
`dropping_session_aborts_intercept_workers_and_kills_spawned_child`
- `cargo test -p codex-core
shell_zsh_fork_prefers_shell_command_over_unified_exec`
- `cargo test -p codex-core --test all
shell_zsh_fork_prompts_for_skill_script_execution`


---
[//]: # (BEGIN SAPLING FOOTER)
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* #13432
* __->__ #13392

codex-rs/core/src/unified_exec/mod.rs

@@ -49,6 +49,10 @@ pub(crate) fn set_deterministic_process_ids_for_tests(enabled: bool) {
 }
 
 pub(crate) use errors::UnifiedExecError;
+pub(crate) use process::NoopSpawnLifecycle;
+#[cfg(unix)]
+pub(crate) use process::SpawnLifecycle;
+pub(crate) use process::SpawnLifecycleHandle;
 pub(crate) use process::UnifiedExecProcess;
 
 pub(crate) const MIN_YIELD_TIME_MS: u64 = 250;