Delivery API: Only add default strategy if delivery API is not registered. (#20982)

* Only add if not already present

* Update src/Umbraco.Cms.Api.Management/DependencyInjection/WebhooksBuilderExtensions.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Kenn Jacobsen <kja@umbraco.dk>

Author: 3 people

src/Umbraco.Cms.Api.Management/DependencyInjection/WebhooksBuilderExtensions.cs

@@ -1,4 +1,4 @@
-using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using Umbraco.Cms.Api.Common.Accessors;
 using Umbraco.Cms.Api.Common.Rendering;
 using Umbraco.Cms.Api.Management.Factories;
@@ -16,9 +16,10 @@ internal static IUmbracoBuilder AddWebhooks(this IUmbracoBuilder builder)
         builder.Services.AddUnique<IWebhookPresentationFactory, WebhookPresentationFactory>();
         builder.AddMapDefinition<WebhookEventMapDefinition>();
 
-        // deliveryApi will overwrite these more basic ones.
-        builder.Services.AddScoped<IOutputExpansionStrategy, ElementOnlyOutputExpansionStrategy>();
-        builder.Services.AddSingleton<IOutputExpansionStrategyAccessor, RequestContextOutputExpansionStrategyAccessor>();
+        // We have to use TryAdd here, as if they are registered by the delivery API, we don't want to register them
+        // Delivery API will also overwrite these IF it is enabled.
+        builder.Services.TryAddScoped<IOutputExpansionStrategy, ElementOnlyOutputExpansionStrategy>();
+        builder.Services.TryAddSingleton<IOutputExpansionStrategyAccessor, RequestContextOutputExpansionStrategyAccessor>();
 
         return builder;
     }