feat: gate unified exec zsh fork composition (#24979)

## Why

`shell_zsh_fork` and unified exec need to remain independently
controllable for enterprise rollouts, but we also need a third mode that
composes them. That composed mode is intended to preserve unified exec
command lifecycle support while letting the zsh fork provide more
accurate `execv(2)` interception.

Enabling `unified_exec_zsh_fork` by itself is intentionally not
sufficient. It is a composition gate, not a dependency-enabling
shortcut:

- `unified_exec` selects the PTY-backed unified exec tool.
- `shell_zsh_fork` opts into the zsh fork backend.
- `unified_exec_zsh_fork` only allows those two already-enabled modes to
be composed so local zsh unified exec commands can launch through the
zsh fork.

This separation is deliberate. Enterprises and staged rollouts must be
able to enable or disable unified exec and zsh-fork independently. If
`unified_exec_zsh_fork` implied either dependency, then enabling one
under-development composition flag would silently activate a shell
backend that the configured feature set left disabled.

This PR introduces only the configuration and planning gate for that
composition. Existing `shell_zsh_fork` behavior continues to use the
standalone shell tool unless the new composition feature is explicitly
enabled alongside both dependencies.

## What Changed

- Added the under-development feature flag `unified_exec_zsh_fork`.
- Added `UnifiedExecFeatureMode` so the three input feature flags
collapse into `Disabled`, `Direct`, or `ZshFork` mode before tool
planning.
- Updated tool selection so zsh-fork composition requires
`unified_exec`, `shell_zsh_fork`, and `unified_exec_zsh_fork`.
- Kept the existing standalone zsh-fork shell tool behavior when only
`shell_zsh_fork` is enabled.
- Updated config schema output for the new feature flag.

## Verification

- Added feature and tool-config coverage for the new gate.
- Added planner coverage proving `shell_zsh_fork` remains standalone
until composition is explicitly enabled.
- Ran focused tests for `codex-features`, `codex-tools`, and the
affected `codex-core` planner case.





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[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/24979).
* #24982
* #24981
* #24980
* __->__ #24979

codex-rs/core/src/session/turn_context.rs

@@ -480,10 +480,8 @@ impl Session {
         let provider_for_context = create_model_provider(provider, auth_manager);
         let session_telemetry_for_context = session_telemetry;
         let available_models = models_manager.try_list_models().unwrap_or_default();
-        let shell_command_backend =
-            shell_command_backend_for_features(per_turn_config.features.get());
         let unified_exec_shell_mode = UnifiedExecShellMode::for_session(
-            shell_command_backend,
+            codex_tools::unified_exec_feature_mode_for_features(per_turn_config.features.get()),
             crate::tools::tool_user_shell_type(user_shell),
             shell_zsh_path,
             main_execve_wrapper_exe,