DEV: Format `UserStatus#ends_at` as a ISO8601 timestamp (#23796)

…as we do when publishing a mesage bus update: 07c93918ec/app/models/user.rb (L871-L871)

app/serializers/user_status_serializer.rb

@@ -3,6 +3,10 @@
 class UserStatusSerializer < ApplicationSerializer
   attributes :description, :emoji, :ends_at, :message_bus_last_id
 
+  def ends_at
+    object.ends_at&.iso8601
+  end
+
   def message_bus_last_id
     MessageBus.last_id("/user-status")
   end

spec/requests/user_status_controller_spec.rb

@@ -28,7 +28,7 @@ RSpec.describe UserStatusController do
       it "returns user status" do
         status = "off to dentist"
         status_emoji = "tooth"
-        ends_at = "2100-01-01T18:00:00.000Z"
+        ends_at = "2100-01-01T18:00:00Z"
         user.set_status!(status, status_emoji, DateTime.parse(ends_at))
 
         get "/user-status.json"

spec/serializers/user_status_serializer_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+RSpec.describe UserStatusSerializer do
+  fab!(:user) { Fabricate(:user) }
+  fab!(:user_status) do
+    Fabricate(
+      :user_status,
+      user: user,
+      set_at: Time.parse("2023-09-29T02:20:00Z"),
+      ends_at: Time.parse("2023-09-29T03:25:00Z"),
+    )
+  end
+
+  describe "#ends_at" do
+    it "is formatted as a ISO8601 timestamp" do
+      serialized = described_class.new(user_status, scope: Guardian.new(user), root: false).as_json
+      expect(serialized[:ends_at]).to eq("2023-09-29T03:25:00Z")
+    end
+  end
+end