Use pages url path in page preview for external sites

If the page preview renders an external site instead of the internal one we pass the url-path instead of the urlname so we make sure that root page urls are ` /` and the language code is prefixed.

lib/alchemy/admin/preview_url.rb

@@ -43,7 +43,7 @@ def url_for(page)
         if @preview_config && uri
           uri_class.build(
             host: uri.host,
-            path: "/#{page.urlname}",
+            path: page.url_path,
             userinfo: userinfo,
           ).to_s
         else

spec/libraries/admin/preview_url_spec.rb

@@ -121,6 +121,20 @@
           end
         end
       end
+
+      context "with page being the language root page" do
+        let(:page) { create(:alchemy_page, :language_root) }
+
+        before do
+          stub_alchemy_config(:preview, {
+            "host" => "https://www.example.com",
+          })
+        end
+
+        it "returns the preview url without urlname" do
+          is_expected.to eq "https://www.example.com/"
+        end
+      end
     end
   end
 end