[codex] Ignore local curated plugins when remote catalog is active (#29765)

## Summary

- suppress configured `openai-curated` plugins when the remote plugin
feature is enabled and auth uses the Codex backend
- preserve `openai-api-curated` and non-Codex-backend behavior while
including remote catalog activation in the plugin load cache key
- add core plugin coverage and an app-server integration test for
runtime feature enablement

## Why

The Codex app enables remote plugins through process-local runtime
feature enablement, which can happen after app-server startup tasks have
already observed legacy local plugin state. The existing conflict logic
only preferred a remote plugin when the same plugin was already
installed remotely, so a configured legacy-only plugin could continue
exposing skills and other capabilities from `openai-curated`.

## Impact

When the remote catalog is active, legacy `openai-curated` plugins no
longer contribute skills, MCP servers, apps, or hooks. Remote installed
plugins continue to load normally, and `openai-api-curated` remains
unaffected. This does not change remote fetch, bundle sync, or uninstall
behavior.

## Validation

- `just test -p codex-core-plugins
remote_global_catalog_ignores_local_curated_plugins
remote_plugin_feature_keeps_local_curated_without_codex_backend`
- `just test -p codex-app-server
runtime_remote_plugin_enablement_excludes_local_curated_plugin_skills`
- `just fmt`
- `git diff --check`

codex-rs/core-plugins/src/loader_tests.rs

@@ -162,14 +162,14 @@ enabled = true
         &store,
         /*plugin_skill_snapshots*/ None,
         Some(Product::Codex),
-        /*prefer_remote_curated_conflicts*/ false,
+        /*remote_global_catalog_active*/ false,
     )
     .await;
     let hooks_only = load_plugins_from_layer_stack_with_scope(
         &stack,
         HashMap::new(),
         &store,
-        /*prefer_remote_curated_conflicts*/ false,
+        /*remote_global_catalog_active*/ false,
         PluginLoadScope::HooksOnly,
     )
     .await;