Fix config test on macOS (#11579)

When running these tests locally, you may have system-wide config or
requirements files. This makes the tests ignore these files.

codex-rs/core/src/config_loader/tests.rs

@@ -240,7 +240,9 @@ async fn returns_empty_when_all_layers_missing() {
     let overrides = LoaderOverrides {
         managed_config_path: Some(managed_path),
         #[cfg(target_os = "macos")]
-        managed_preferences_base64: None,
+        // Force managed preferences to resolve as empty so this test does not
+        // inherit non-empty machine-specific managed state.
+        managed_preferences_base64: Some(String::new()),
         macos_managed_config_requirements_base64: None,
     };