feat: sync guard - Blockieren alter Clients beim sync

LehrerApp.Desktop.Tests/SyncStatusViewModelTests.cs

@@ -67,6 +67,19 @@ public sealed class SyncStatusViewModelTests
         Assert.False(fired);
     }
 
+    [Fact]
+    public async Task InkompatibleSyncVersion_ZeigtKlareUpdateMeldung()
+    {
+        using var temp = new TempSyncEngine(new IncompatibleVersionStubHandler());
+        var vm = new SyncStatusViewModel(temp.Engine);
+
+        await temp.Engine.SyncNowAsync();
+
+        Assert.True(vm.IsIncompatibleVersion);
+        Assert.Equal("Update erforderlich", vm.StatusText);
+        Assert.Contains("Bitte aktualisiere die LehrerApp", SyncStatusViewModel.IncompatibleVersionMessage);
+    }
+
     private sealed class PullEventStubHandler : HttpMessageHandler
     {
         protected override Task<HttpResponseMessage> SendAsync(
@@ -100,6 +113,17 @@ public sealed class SyncStatusViewModelTests
             });
     }
 
+    private sealed class IncompatibleVersionStubHandler : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            var response = new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+            response.Headers.Add(SyncProtocol.VersionHeaderName, "2");
+            return Task.FromResult(response);
+        }
+    }
+
     private sealed class TempSyncEngine : IDisposable
     {
         private readonly string _queuePath = Path.Combine(