Combine step_nav and related_step_navs list to search both lists

This is needed so that when the active step by step is searched for
it searches related_to_step_navs and step_navs list to make sure the
active step by step is shown correctly

lib/govuk_publishing_components/presenters/page_with_step_by_step_navigation.rb

@@ -73,7 +73,8 @@ def active_step_by_step?
       end
 
       def active_step_by_step
-        @active_step_navs ||= step_navs.select { |step_nav| step_nav.content_id == active_step_nav_content_id }
+        step_navs_list = step_navs_combined_list
+        @active_step_navs ||= step_navs_list.select { |step_nav| step_nav.content_id == active_step_nav_content_id }
         @active_step_navs.first
       end
 
@@ -94,6 +95,13 @@ def steps
         @steps ||= step_nav[:steps]
       end
 
+      def step_navs_combined_list
+        step_nav_list = []
+        step_nav_list += step_navs if step_navs.any?
+        step_nav_list += related_to_step_navs if related_to_step_navs.any?
+        step_nav_list
+      end
+
       def parsed_step_navs
         content_item.dig("links", "part_of_step_navs").to_a
       end

spec/lib/presenters/page_with_step_by_step_navigation_spec.rb

@@ -345,6 +345,42 @@
       expect(step_nav_helper.also_part_of_step_nav.first[:tracking_id]).to eq('aaaa-bbbb')
       expect(step_nav_helper.show_also_part_of_step_nav?).to be true
     end
+
+    it "shows related to step nav when a step by step is active" do
+      step_nav = {
+        "content_id" => "cccc-dddd",
+        "title" => "Learn to spacewalk: small step by giant leap",
+        "base_path" => "/learn-to-spacewalk"
+      }
+      content_item = {
+        "title" => "Book a session in the vomit comet",
+        "document_type" => "transaction",
+        "links" => {
+          "related_to_step_navs" => [step_nav],
+        }
+      }
+      step_nav_helper = described_class.new(content_item, "/driving-lessons-learning-to-drive", "step-by-step-nav" => "cccc-dddd")
+      expect(step_nav_helper.active_step_by_step?).to eq(true)
+      expect(step_nav_helper.also_part_of_step_nav.count).to eq(0)
+    end
+
+    it "does not shows related to step nav when a step by step is not active" do
+      step_nav = {
+        "content_id" => "cccc-dddd",
+        "title" => "Learn to spacewalk: small step by giant leap",
+        "base_path" => "/learn-to-spacewalk"
+      }
+      content_item = {
+        "title" => "Book a session in the vomit comet",
+        "document_type" => "transaction",
+        "links" => {
+          "related_to_step_navs" => [step_nav],
+        }
+      }
+      step_nav_helper = described_class.new(content_item, "/driving-lessons-learning-to-drive")
+      expect(step_nav_helper.active_step_by_step?).to eq(false)
+      expect(step_nav_helper.show_also_part_of_step_nav?).to be false
+    end
   end
 
   def payload_for(schema, content_item)