[StoreKit] Update to Xcode 26 beta 1-5. (#23486)

Author: rolfbjarne

src/storekit.cs

@@ -12,6 +12,7 @@
 
 using System.ComponentModel;
 
+using BackgroundAssets;
 using ObjCRuntime;
 using Foundation;
 using CoreFoundation;
@@ -1756,4 +1757,9 @@ interface SKAdImpression {
 		string Version { get; set; }
 
 	}
+
+	[iOS (26, 0), Mac (26, 0), TV (26, 0), MacCatalyst (26, 0)]
+	[Protocol (BackwardsCompatibleCodeGeneration = false)]
+	interface SKDownloaderExtension : BAManagedDownloaderExtension {
+	}
 }