[plugins] Install MCPs when calling plugin/install (#15195)

- [x] Auth MCPs when installing plugins.

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

@@ -36,6 +36,7 @@ pub use manager::PluginsManager;
 pub use manager::RemotePluginSyncResult;
 pub use manager::installed_plugin_telemetry_metadata;
 pub use manager::load_plugin_apps;
+pub use manager::load_plugin_mcp_servers;
 pub(crate) use manager::plugin_namespace_for_skill_path;
 pub use manager::plugin_telemetry_metadata_from_root;
 pub use manifest::PluginManifestInterface;