Ensure additional preview URLs work on non-variant content

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

#14773 added UI for the "additional preview URLs" feature implemented in #14479. Unfortunately, the "additional preview URLs" UI currently does not work with culture invariant content.

This PR fixes that issue. It is purely a client-side issue which has to do with the active variant election. See the code change 😄

Testing this PR

Try adding a SendingContentNotification notification handler like the following, and ensure that the generated additional preview URLs are displayed for both variant and invariant content.

using Umbraco.Cms.Core.Notifications;
using Umbraco.Cms.Core.PublishedCache;

namespace Umbraco.Cms.Web.UI.Custom;

public class AdditionalPreviewUrlsNotificationHandler : INotificationHandler<SendingContentNotification>
{
    private readonly IPublishedSnapshotAccessor _publishedSnapshotAccessor;

    public AdditionalPreviewUrlsNotificationHandler(IPublishedSnapshotAccessor publishedSnapshotAccessor)
        => _publishedSnapshotAccessor = publishedSnapshotAccessor;

    public void Handle(SendingContentNotification notification)
    {
        foreach (ContentVariantDisplay variantDisplay in notification.Content.Variants.Where(variant => variant.PublishDate.HasValue))
        {
            IPublishedSnapshot publishedSnapshot = _publishedSnapshotAccessor.GetRequiredPublishedSnapshot();
            var route = publishedSnapshot.Content?.GetRouteById(true, notification.Content.Id, variantDisplay.Language?.IsoCode);
            if (route == null)
            {
                continue;
            }

            route = route.TrimStart('/');

            variantDisplay.AdditionalPreviewUrls = new[]
            {
                new NamedUrl
                {
                    Name = $"Environment One{(variantDisplay.Language != null ? $" ({variantDisplay.Language.Name})" : null)}",
                    Url = $"https://environment.one/{route}?culture={variantDisplay.Language?.IsoCode}"
                },
                new NamedUrl
                {
                    Name = $"Environment Two{(variantDisplay.Language != null ? $" ({variantDisplay.Language.Name})" : null)}",
                    Url = $"https://environment.two/{route}?culture={variantDisplay.Language?.IsoCode}"
                }
            };
        }
    }
}

public class AdditionalPreviewUrlsNotificationHandlerComposer : IComposer
{
    public void Compose(IUmbracoBuilder builder)
        => builder.AddNotificationHandler<SendingContentNotification, AdditionalPreviewUrlsNotificationHandler>();
}