fix: set IsVisible=true in OnClipboardChanged tests, update global.json SDK version

Author: GokulMV

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.0",
+    "version": "8.0.420",
     "rollForward": "latestMinor"
   }
 }

tests/ClipHive.Tests/Unit/SidebarViewModelTests.cs

@@ -156,6 +156,7 @@ public async Task OnClipboardChanged_NewContent_TriggersLoadAsync()
         var newItem = MakeItem(42, "new content");
         var (vm, storage, _) = CreateSut(new[] { newItem });
 
+        vm.IsVisible = true;
         vm.OnClipboardChanged("new content");
 
         // Allow the fire-and-forget LoadAsync to complete.
@@ -178,6 +179,7 @@ public async Task OnClipboardChanged_DuplicateContent_LoadsFromStorage()
         };
         var (vm, storage, _) = CreateSut(items);
 
+        vm.IsVisible = true;
         vm.OnClipboardChanged("duplicate");
 
         await Task.Delay(50);