Add show_finder_link? method to SpecialistPublisherPresenter

Audit trail:
specialist_document_presenter.rb: https://github.com/alphagov/government-frontend/blob/c6eead39b29192e692c802c9b5e8bc234e16692c/app/presenters/specialist_document_presenter.rb#L49-L53

app/models/specialist_document.rb

@@ -1,10 +1,11 @@
 class SpecialistDocument < ContentItem
-  attr_reader :continuation_link, :headers, :will_continue_on
+  attr_reader :continuation_link, :finder, :headers, :will_continue_on
 
   def initialize(content_store_response)
     super(content_store_response)
 
     @continuation_link = content_store_hash.dig("details", "metadata", "continuation_link")
+    @finder = content_store_hash.dig("links", "finder", 0)
     @will_continue_on = content_store_hash.dig("details", "metadata", "will_continue_on")
 
     @headers = headers_list(content_store_hash.dig("details", "headers"))

app/presenters/specialist_document_presenter.rb

@@ -3,9 +3,17 @@ def show_contents_list?
     content_item.headers.present? && level_two_headings?
   end
 
+  def show_finder_link?
+    content_item.finder.present? && statutory_instrument?
+  end
+
 private
 
   def level_two_headings?
     content_item.headers.any? { |header| header[:level] == 2 }
   end
+
+  def statutory_instrument?
+    content_item.document_type == "statutory_instrument"
+  end
 end

spec/presenter/specialist_document_presenter_spec.rb

@@ -14,4 +14,20 @@
       expect(described_class.new(content_item).show_contents_list?).to be true
     end
   end
+
+  describe "#show_finder_link?" do
+    it "returns true when document type is statutory instrument" do
+      content_store_response = GovukSchemas::Example.find("specialist_document", example_name: "eu-withdrawal-act-2018-statutory-instruments")
+      content_item = SpecialistDocument.new(content_store_response)
+
+      expect(described_class.new(content_item).show_finder_link?).to be true
+    end
+
+    it "returns false when document type is not a statutory instrument" do
+      content_store_response = GovukSchemas::Example.find("specialist_document", example_name: "aaib-reports")
+      content_item = SpecialistDocument.new(content_store_response)
+
+      expect(described_class.new(content_item).show_finder_link?).to be false
+    end
+  end
 end