test(app): composer item test installation carries the subscribe main added to InstalledPlugin

Main's plugin streaming (#8527) made subscribe a required field of InstalledPlugin; the
composer item model test builds its own installation and predates it.

packages/app/src/plugins/composer-items/model.test.ts

@@ -20,6 +20,9 @@ function installed(): InstalledPlugin {
     queryClient: new QueryClient(),
     paseo: {} as InstalledPlugin["paseo"],
     invoke: async () => undefined,
+    subscribe: () => {
+      throw new Error("Subscriptions are not used here");
+    },
     cleanup: () => {},
     settingsScreens: [],
     surfaces: [],