FIX: Add back missing API key 'peek' step (#30683)

In #30096 we converted the API keys UI to follow the new admin UI guidelines.

During this conversion, the step where you get a chance to copy the API key after creating, was lost due to a rebase mistake.

This re-introduces it.

spec/system/admin_api_keys_spec.rb

@@ -22,6 +22,10 @@ describe "Admin API Keys Page", type: :system do
     api_keys_page.visit_page
     api_keys_page.add_api_key(description: "Second Integration")
 
+    expect(api_keys_page).to have_generated_api_key
+
+    api_keys_page.click_continue
+
     expect(api_keys_page).to have_api_key_listed("Second Integration")
   end