add optional publishing_app param in PublishIntentPresenter

we want to send Publish Intents for Host Content (e.g. Editions that
are using embedded content/Content Blocks) [1] that is published
by other apps - this allows an optional app to be passed in, but retains
the default Whitehall value in order not to break existing functionality.

[1] #9447

app/presenters/publishing_api/publish_intent_presenter.rb

@@ -1,14 +1,15 @@
 module PublishingApi
   class PublishIntentPresenter
-    def initialize(base_path, publish_timestamp)
+    def initialize(base_path, publish_timestamp, publishing_app = Whitehall::PublishingApp::WHITEHALL)
       @base_path = base_path
       @publish_timestamp = publish_timestamp
+      @publishing_app = publishing_app
     end
 
     def as_json
       {
         publish_time: @publish_timestamp,
-        publishing_app: Whitehall::PublishingApp::WHITEHALL,
+        publishing_app: @publishing_app,
         rendering_app: Whitehall::RenderingApp::GOVERNMENT_FRONTEND,
         routes: [{ path: @base_path, type: "exact" }],
       }

test/unit/app/presenters/publishing_api/publish_intent_presenter_test.rb

@@ -0,0 +1,36 @@
+require "test_helper"
+
+module PublishingApi
+  class PublishIntentPresenterTest < ActiveSupport::TestCase
+    test "it returns Whitehall as default publisher" do
+      base_path = "/example-path"
+      publish_timestamp = Time.zone.now.to_s
+
+      presenter = PublishingApi::PublishIntentPresenter.new(base_path, publish_timestamp)
+      expected_hash = {
+        publish_time: publish_timestamp,
+        publishing_app: Whitehall::PublishingApp::WHITEHALL,
+        rendering_app: Whitehall::RenderingApp::GOVERNMENT_FRONTEND,
+        routes: [{ path: base_path, type: "exact" }],
+      }
+
+      assert_equal presenter.as_json, expected_hash
+    end
+
+    test "it returns publishing app if provided" do
+      base_path = "/example-path"
+      publish_timestamp = Time.zone.now.to_s
+      publishing_app = "example-publishing-app"
+
+      presenter = PublishingApi::PublishIntentPresenter.new(base_path, publish_timestamp, publishing_app)
+      expected_hash = {
+        publish_time: publish_timestamp,
+        publishing_app: "example-publishing-app",
+        rendering_app: Whitehall::RenderingApp::GOVERNMENT_FRONTEND,
+        routes: [{ path: base_path, type: "exact" }],
+      }
+
+      assert_equal presenter.as_json, expected_hash
+    end
+  end
+end