Allow trusted symlinks beneath CODEX_HOME on macOS (#42716)

## What changed

- Add the macOS-only `allow_symlinked_codex_home` user setting. When enabled in the execution host's `$CODEX_HOME/config.toml`, writable roots at or beneath that home may traverse symlinks.
- Keep the exception disabled by default and prevent project config, command-line overrides, and ignored user config from enabling it. Other writable roots remain subject to symlink checks.
- Propagate the resolved setting through local, interactive, and exec-server sandbox paths, including `CODEX_HOME` aliases.
- Explain the opt-out in symlink rejection errors, including that it trusts targets outside `CODEX_HOME` and targets that change between commands.

## Testing

- Add macOS coverage for shell commands, patches, filesystem helpers, process execution, and interactive startup with enabled, disabled, aliased, and out-of-scope homes.

GitOrigin-RevId: 99fcdf611200c9e1b7713cf06f7fdea5bfa7f089

codex-rs/exec-server/tests/common/exec_server.rs

@@ -102,7 +102,13 @@ impl ExecServerHarness {
         command.stdout(Stdio::piped());
         command.stderr(Stdio::inherit());
         command.kill_on_drop(true);
-        command.env("CODEX_HOME", codex_home.path());
+        if !command
+            .as_std()
+            .get_envs()
+            .any(|(key, value)| key == "CODEX_HOME" && value.is_some())
+        {
+            command.env("CODEX_HOME", codex_home.path());
+        }
         let mut child = command.spawn()?;
 
         let websocket_url = read_listen_url_from_stdout(&mut child).await?;