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

- [x] Auth MCPs when installing plugins.

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

@@ -1660,6 +1660,22 @@ pub fn plugin_telemetry_metadata_from_root(
     }
 }
 
+pub fn load_plugin_mcp_servers(plugin_root: &Path) -> HashMap<String, McpServerConfig> {
+    let Some(manifest) = load_plugin_manifest(plugin_root) else {
+        return HashMap::new();
+    };
+
+    let mut mcp_servers = HashMap::new();
+    for mcp_config_path in plugin_mcp_config_paths(plugin_root, &manifest.paths) {
+        let plugin_mcp = load_mcp_servers_from_file(plugin_root, &mcp_config_path);
+        for (name, config) in plugin_mcp.mcp_servers {
+            mcp_servers.entry(name).or_insert(config);
+        }
+    }
+
+    mcp_servers
+}
+
 pub fn installed_plugin_telemetry_metadata(
     codex_home: &Path,
     plugin_id: &PluginId,

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;