Pass AB params to Search::Query

The ab_params is only needed to determine which version of search-api to call. It will not need be needed in search-api itself.

Rather than updating all of the tests to stub passing in the ab_params the ab_params argument is no longer being passed to the QueryBuilder because that would require updating all of the tests to stub passing the ab_params to search-api.

Uses "anything" as an RSpec matcher. This is a positional argument that allows you specify "anything"
if you don't care what the other arguments are.

app/controllers/ab_tests/vertex_search_ab_testable.rb

@@ -17,4 +17,7 @@ def set_requested_variant
     @requested_variant.configure_response(response)
   end
 
+  def ab_params
+    { vertex: @requested_variant.variant_name }
+  end
 end

app/controllers/finders_controller.rb

@@ -119,7 +119,7 @@ def initialize_search_query(is_for_feed: false)
       content_item,
       filter_params,
       override_sort_for_feed: is_for_feed,
-      ab_params: {},
+      ab_params: page_under_test? ? ab_params : {},
     )
   end
 

app/lib/search/query.rb

@@ -83,7 +83,6 @@ def fetch_search_response(content_item)
       queries = QueryBuilder.new(
         finder_content_item: content_item,
         params: filter_params,
-        ab_params:,
         override_sort_for_feed:,
       ).call
 

spec/controllers/finders_controller_spec.rb

@@ -140,6 +140,19 @@
             expect(response.header["Vary"]).not_to eq("GOVUK-ABTest-VertexSearch")
             expect(response.body).not_to include("VertexSearch:#{variant}")
           end
+
+          it "sends ab_params to search query if page is being tested" do
+            stub_content_store_has_item(
+              "/search/all",
+              all_content_finder,
+            )
+
+            @request.headers["GOVUK-ABTest-VertexSearch"] = variant
+
+            expect(Search::Query).to receive(:new).with(anything, anything, hash_including(ab_params: { vertex: variant })).and_call_original
+
+            get :show, params: { slug: "search/all" }
+          end
         end
 
         it "should render the page without an AB test variant for search" do