FIX: Always preload admin plugin list for admin in sidebar (#25606)

When we show the links to installed plugins in the admin
sidebar (for plugins that have custom admin routes) we were
previously only doing this if you opened /admin, not if you
navigated there from the main forum. We should just always
preload this data if the user is admin.

This commit also changes `admin_sidebar_enabled_groups` to
not be sent to the client as part of ongoing efforts to
not check groups on the client, since not all a user's groups
may be serialized.

plugins/chat/spec/requests/application_controller_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe ApplicationController do
+  fab!(:user)
+  fab!(:admin)
+
+  def preloaded_json
+    JSON.parse(
+      Nokogiri::HTML5.fragment(response.body).css("div#data-preloaded").first["data-preloaded"],
+    )
+  end
+
+  before do
+    SiteSetting.chat_enabled = true
+    SiteSetting.chat_allowed_groups = Group::AUTO_GROUPS[:trust_level_0]
+  end
+
+  context "when user is admin" do
+    it "has correctly loaded preloaded data for enabledPluginAdminRoutes" do
+      sign_in(admin)
+      get "/latest"
+      expect(JSON.parse(preloaded_json["enabledPluginAdminRoutes"])).to include(
+        { "label" => "chat.admin.title", "location" => "chat" },
+      )
+    end
+  end
+
+  context "when user is not admin" do
+    it "does not include preloaded data for enabledPluginAdminRoutes" do
+      sign_in(user)
+      get "/latest"
+      expect(preloaded_json["enabledPluginAdminRoutes"]).to eq(nil)
+    end
+  end
+end