Resolve model-provided shells by type (#39607)

## Why

A model-provided shell path should select the requested shell type without
allowing that path to determine which executable Codex runs.

## What changed

- Resolve model-provided shells through Codex's normal shell discovery and
  fallback logic after detecting their type.
- Keep the configured packaged zsh executable when the zsh-fork feature is
  enabled and the file exists.
- Update shell, unified exec, and network approval expectations to use the
  resolved local executable and arguments.

GitOrigin-RevId: ebe6f7eec2cfd1c0548d5bf1a26b7a30dba02cc2

codex-rs/core/tests/suite/mod.rs

@@ -16,21 +16,6 @@ use ctor::ctor;
 #[ctor]
 pub static CODEX_ALIASES_TEMP_DIR: Option<TestBinaryDispatchGuard> = {
     configure_test_binary_dispatch("codex-core-tests", |exe_name, argv1| {
-        #[cfg(windows)]
-        if exe_name.eq_ignore_ascii_case("powershell.exe")
-            || exe_name.eq_ignore_ascii_case("powershell.evil")
-            || exe_name.eq_ignore_ascii_case("pwsh.exe")
-        {
-            let executable = std::env::current_exe().expect("locate fake PowerShell executable");
-            if executable
-                .with_file_name(".codex-executable-identity-fixture")
-                .is_file()
-            {
-                let marker = executable.with_file_name("attacker-executed");
-                std::fs::write(marker, b"ran").expect("record fake PowerShell execution");
-                std::process::exit(0);
-            }
-        }
         if argv1 == Some(CODEX_CORE_APPLY_PATCH_ARG1) {
             return TestBinaryDispatchMode::DispatchArg0Only;
         }
@@ -77,7 +62,6 @@ mod cyber_exec_policy;
 mod deprecation_notice;
 mod exec;
 mod exec_policy;
-mod executable_identity;
 #[cfg(not(target_os = "windows"))]
 mod extension_sandbox;
 mod external_auth;