check app auth in plugin/install (#13685)

#### What
on `plugin/install`, check if installed apps are already authed on
chatgpt, and return list of all apps that are not. clients can use this
list to trigger auth workflows as needed.

checks are best effort based on `codex_apps` loading, much like
`app/list`.

#### Tests
Added integration tests, tested locally.

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

@@ -15,6 +15,7 @@ pub use manager::PluginInstallError;
 pub use manager::PluginInstallRequest;
 pub use manager::PluginLoadOutcome;
 pub use manager::PluginsManager;
+pub use manager::load_plugin_apps;
 pub(crate) use manager::plugin_namespace_for_skill_path;
 pub(crate) use manifest::load_plugin_manifest;
 pub(crate) use manifest::plugin_manifest_name;