Add test helper to stub no content item returned

It mimics stub_content_store_does_not_have_item in GdsApi adapters.

GdsApi::HTTPErrorResponse params are based on:
https://github.com/alphagov/gds-api-adapters/blob/4d4b2ab6969f333eb1cab94ff8e93d7ff8aeebf1/lib/gds_api/exceptions.rb#L25C1-L34C6

Author: AgaDufrat

README.md

@@ -86,6 +86,10 @@ loader = GovukGraphql::ConditionalContentItemLoader.new(
 
 The decision logic is also exposed via `can_load_from_graphql?`, allowing applications to make the routing decision themselves and implement custom fallback or error-handling behaviour if needed.
 
+## Test helpers
+
+There are also test helpers for stubbing responses in other apps. See [lib/test_helpers.rb](/lib/test_helpers.rb).
+
 ## Licence
 
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).

lib/govuk_content_item_loader/test_helpers.rb

@@ -31,4 +31,28 @@ def stub_conditional_loader_returns_content_item(body, options = {})
 
     response
   end
+
+  # Stubs the loader returns a error response
+  # The following options can be passed in:
+  #
+  #   :status         the HTTP status code for the error. Defaults to 404
+  #   :message        the error message. Defaults to "Not Found"
+  #   :error_details  optional additional error details to attach to the exception
+  #   :http_body      optional raw response body to attach to the exception
+  def stub_conditional_loader_does_not_return_content_item(options = {})
+    status = options.fetch(:status, 404)
+    message = options.fetch(:message, "Not Found")
+    error_details = options[:error_details]
+    http_body = options[:http_body]
+
+    error = GdsApi::HTTPErrorResponse.new(status, message, error_details, http_body)
+
+    loader = instance_double(GovukConditionalContentItemLoader)
+
+    allow(GovukConditionalContentItemLoader).to receive(:new).with(request: anything)
+      .and_return(loader)
+    allow(loader).to receive(:load).and_raise(error)
+
+    error
+  end
 end