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)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/13392).
* #13432
* __->__ #13392

codex-rs/core/src/tools/spec.rs

@@ -44,10 +44,17 @@ pub enum ShellCommandBackendConfig {
     ZshFork,
 }
 
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+pub enum UnifiedExecBackendConfig {
+    Direct,
+    ZshFork,
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct ToolsConfig {
     pub shell_type: ConfigShellToolType,
     shell_command_backend: ShellCommandBackendConfig,
+    pub unified_exec_backend: UnifiedExecBackendConfig,
     pub allow_login_shell: bool,
     pub apply_patch_tool_type: Option<ApplyPatchToolType>,
     pub web_search_mode: Option<WebSearchMode>,
@@ -102,6 +109,12 @@ impl ToolsConfig {
             } else {
                 ShellCommandBackendConfig::Classic
             };
+        let unified_exec_backend =
+            if features.enabled(Feature::ShellTool) && features.enabled(Feature::ShellZshFork) {
+                UnifiedExecBackendConfig::ZshFork
+            } else {
+                UnifiedExecBackendConfig::Direct
+            };
 
         let shell_type = if !features.enabled(Feature::ShellTool) {
             ConfigShellToolType::Disabled
@@ -140,6 +153,7 @@ impl ToolsConfig {
         Self {
             shell_type,
             shell_command_backend,
+            unified_exec_backend,
             allow_login_shell: true,
             apply_patch_tool_type,
             web_search_mode: *web_search_mode,
@@ -2817,6 +2831,10 @@ mod tests {
             tools_config.shell_command_backend,
             ShellCommandBackendConfig::ZshFork
         );
+        assert_eq!(
+            tools_config.unified_exec_backend,
+            UnifiedExecBackendConfig::ZshFork
+        );
     }
 
     #[test]