Standardize shell execution on unified exec (#39772)

## What changed

- Use `exec_command` and `write_stdin` as the shell tool surface.
- Treat legacy `default`, `local`, and `shell_command` model metadata as `unified_exec`.
- Remove obsolete shell-selection configuration and runtime paths while preserving the feature and policy gates for zsh fork execution.

GitOrigin-RevId: d743cbe598630d73052f1fecad680c4cde17977d

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

@@ -647,7 +647,7 @@ impl Session {
         let session_telemetry_for_context = session_telemetry;
         let available_models = models_manager.try_list_models().unwrap_or_default();
         let unified_exec_shell_mode = UnifiedExecShellMode::for_session(
-            codex_tools::unified_exec_feature_mode_for_features(per_turn_config.features.get()),
+            per_turn_config.features.get(),
             crate::tools::tool_user_shell_type(user_shell),
             shell_zsh_path,
             main_execve_wrapper_exe,