FEATURE: allow admins to enable announced experimental features (#29244)

Toggle the button to enable the experimental site setting from "What's new" announcement.

The toggle button is displayed when:
- site setting exists and is boolean;
- potentially required plugin is enabled.

spec/lib/discourse_updates_spec.rb

@@ -248,6 +248,64 @@ RSpec.describe DiscourseUpdates do
       expect(result[1]["title"]).to eq("Whistles")
       expect(result[2]["title"]).to eq("Bells")
     end
+
+    it "correctly shows features with correct boolean experimental site settings" do
+      features_with_versions = [
+        {
+          "emoji" => "🤾",
+          "title" => "Bells",
+          "created_at" => 2.days.ago,
+          "experiment_setting" => "enable_mobile_theme",
+        },
+        {
+          "emoji" => "🙈",
+          "title" => "Whistles",
+          "created_at" => 3.days.ago,
+          "experiment_setting" => "default_theme_id",
+        },
+        {
+          "emoji" => "🙈",
+          "title" => "Confetti",
+          "created_at" => 4.days.ago,
+          "experiment_setting" => "wrong value",
+        },
+      ]
+
+      Discourse.redis.set("new_features", MultiJson.dump(features_with_versions))
+      DiscourseUpdates.last_installed_version = "2.7.0.beta2"
+      result = DiscourseUpdates.new_features
+
+      expect(result.length).to eq(3)
+      expect(result[0]["experiment_setting"]).to eq("enable_mobile_theme")
+      expect(result[1]["experiment_setting"]).to be_nil
+      expect(result[2]["experiment_setting"]).to be_nil
+    end
+
+    it "correctly shows features when related plugins are installed" do
+      Discourse.stubs(:plugins_by_name).returns({ "discourse-ai" => true })
+
+      features_with_versions = [
+        {
+          "emoji" => "🤾",
+          "title" => "Bells",
+          "created_at" => 2.days.ago,
+          "plugin_name" => "discourse-ai",
+        },
+        {
+          "emoji" => "🙈",
+          "title" => "Confetti",
+          "created_at" => 4.days.ago,
+          "plugin_name" => "uninstalled-plugin",
+        },
+      ]
+
+      Discourse.redis.set("new_features", MultiJson.dump(features_with_versions))
+      DiscourseUpdates.last_installed_version = "2.7.0.beta2"
+      result = DiscourseUpdates.new_features
+
+      expect(result.length).to eq(1)
+      expect(result[0]["title"]).to eq("Bells")
+    end
   end
 
   describe "#get_last_viewed_feature_date" do