fix(iOS): drain UISceneConnectionOptions on cold launch (AvaloniaUI#21362) (AvaloniaUI#21394)

When the iOS app uses scene-based lifecycle (iOS 13+), Apple delivers
cold-launch user activities and URL contexts via UISceneConnectionOptions
on scene:willConnectToSession:options: — NOT via the warm-path selectors
scene:continueUserActivity: or scene:openURLContexts:. AvaloniaSceneDelegate
ignored connectionOptions entirely, so cold-launching the app from a
Universal Link or custom-scheme URL dropped the URL on the floor.

The user's subscriber to IActivatableLifetime.Activated then only saw
the base ActivatedEventArgs raised by the foreground transition
(OnLeavingBackground), never a ProtocolActivatedEventArgs with the URL.

Fix: in WillConnect, after the window is created and made key, drain
both connectionOptions.UserActivities and connectionOptions.UrlContexts
through the existing IAvaloniaAppInternalDelegate paths. Also add a
scene:openURLContexts: handler so warm custom-scheme URLs work under
scene lifecycle (previously these were silently dropped too —
application:openURL: on the AppDelegate is not called when scenes are
in use).

Closes AvaloniaUI#21362 (cold path of AvaloniaUI#17600 / AvaloniaUI#18005).

The IAvaloniaAppInternalDelegate interface gains an OpenUrl(NSUrl)
method so the scene delegate can route URL contexts through the same
code path the existing application:openURL:options: AppDelegate handler
uses.

No new tests — the Avalonia.iOS UnitTests and IntegrationTests projects
don't currently exist in the repo. Manual verification steps in the
issue: build any Avalonia.iOS app with Associated Domains entitlement
+ AASA served, force-quit, tap a Universal Link, observe the
Activated event fires with ProtocolActivatedEventArgs (not base
ActivatedEventArgs). Happy to add a test if maintainers can point at
a scene-lifecycle test fixture to model from.

Author: koblavi

src/iOS/Avalonia.iOS/AvaloniaAppDelegate.cs

@@ -15,6 +15,7 @@ public interface IAvaloniaAppDelegate
     internal interface IAvaloniaAppInternalDelegate
     {
         bool ContinueUserActivity(NSUserActivity userActivity);
+        bool OpenUrl(NSUrl url);
     }
 
     public class AvaloniaAppDelegate<TApp> : UIResponder, IUIApplicationDelegate, IAvaloniaAppDelegate, IAvaloniaAppInternalDelegate
@@ -87,6 +88,9 @@ private void CreateAndInitWindow(SingleViewLifetime lifetime)
 
         [Export("application:openURL:options:")]
         public bool OpenUrl(UIApplication app, NSUrl url, NSDictionary options)
+            => ((IAvaloniaAppInternalDelegate)this).OpenUrl(url);
+
+        bool IAvaloniaAppInternalDelegate.OpenUrl(NSUrl url)
         {
             if (Uri.TryCreate(url.ToString(), UriKind.Absolute, out var uri))
             {
@@ -98,7 +102,7 @@ public bool OpenUrl(UIApplication app, NSUrl url, NSDictionary options)
                 else
 #endif
                 {
-                    _onActivated?.Invoke(this, new ProtocolActivatedEventArgs(uri));   
+                    _onActivated?.Invoke(this, new ProtocolActivatedEventArgs(uri));
                 }
                 return true;
             }

src/iOS/Avalonia.iOS/AvaloniaSceneDelegate.cs

@@ -1,4 +1,6 @@
-using Foundation;
+using System;
+using Avalonia.Controls.ApplicationLifetimes;
+using Foundation;
 using UIKit;
 
 namespace Avalonia.iOS;
@@ -22,6 +24,18 @@ scene is not UIWindowScene windowScene ||
         InitWindow(Window, lifetime);
 
         Window.MakeKeyAndVisible();
+
+        // Cold-launch activation: when iOS launches the app to handle a
+        // Universal Link, a Handoff, or a custom-scheme URL, the
+        // payload is delivered via the connectionOptions parameter —
+        // NOT via the warm-path scene:continueUserActivity: or
+        // scene:openURLContexts: selectors. Without draining the
+        // options here, the URL is silently dropped and subscribers to
+        // IActivatableLifetime.Activated only see the base
+        // ActivatedEventArgs from the foreground transition. Fixes the
+        // cold path of #17600 that PR #18005 only addressed for the
+        // warm case.
+        DispatchConnectionOptions(connectionOptions);
     }
 
     [Export("scene:continueUserActivity:")]
@@ -31,6 +45,35 @@ public void ContinueUserActivity(UIScene scene, NSUserActivity userActivity)
         appDelegate?.ContinueUserActivity(userActivity);
     }
 
+    [Export("scene:openURLContexts:")]
+    public void OpenUrlContexts(UIScene scene, NSSet<UIOpenUrlContext> urlContexts)
+    {
+        var appDelegate = UIApplication.SharedApplication.Delegate as IAvaloniaAppInternalDelegate;
+        if (appDelegate is null || urlContexts is null)
+            return;
+        foreach (var ctx in urlContexts)
+            appDelegate.OpenUrl(ctx.Url);
+    }
+
+    private static void DispatchConnectionOptions(UISceneConnectionOptions connectionOptions)
+    {
+        var appDelegate = UIApplication.SharedApplication.Delegate as IAvaloniaAppInternalDelegate;
+        if (appDelegate is null)
+            return;
+
+        if (connectionOptions.UserActivities is { } activities)
+        {
+            foreach (NSUserActivity activity in activities)
+                appDelegate.ContinueUserActivity(activity);
+        }
+
+        if (connectionOptions.UrlContexts is { } urlContexts)
+        {
+            foreach (var ctx in urlContexts)
+                appDelegate.OpenUrl(ctx.Url);
+        }
+    }
+
     internal static void InitWindow(UIWindow window, SingleViewLifetime lifetime)
     {
         var view = new AvaloniaView();