feat(01-06): implement FeatureViewModelBase with async/cancel/progress pattern

- Add ProgressUpdatedMessage ValueChangedMessage for StatusBar live updates
- Add FeatureViewModelBase with CancellationTokenSource lifecycle, IsRunning,
  IProgress<OperationProgress>, OperationCanceledException handling
- Add 6 unit tests covering lifecycle, progress, cancellation, error handling
  and CanExecute guard

SharepointToolbox.Tests/ViewModels/FeatureViewModelBaseTests.cs

@@ -1,7 +1,125 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.ViewModels;
+
 namespace SharepointToolbox.Tests.ViewModels;
 
+[Trait("Category", "Unit")]
 public class FeatureViewModelBaseTests
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-06")]
-    public void CancelCommand_Cancels_RunningOperation() { }
+    private class TestViewModel : FeatureViewModelBase
+    {
+        public TestViewModel() : base(NullLogger<FeatureViewModelBase>.Instance) { }
+        public Func<CancellationToken, IProgress<OperationProgress>, Task>? OperationFunc { get; set; }
+        protected override Task RunOperationAsync(CancellationToken ct, IProgress<OperationProgress> progress)
+            => OperationFunc?.Invoke(ct, progress) ?? Task.CompletedTask;
+    }
+
+    [Fact]
+    public async Task IsRunning_IsTrueWhileOperationExecutes_ThenFalseAfterCompletion()
+    {
+        var vm = new TestViewModel();
+        var tcs = new TaskCompletionSource<bool>();
+        bool wasRunningDuringOperation = false;
+
+        vm.OperationFunc = async (ct, p) =>
+        {
+            wasRunningDuringOperation = vm.IsRunning;
+            await tcs.Task;
+        };
+
+        var runTask = vm.RunCommand.ExecuteAsync(null);
+        // Give run task time to start
+        await Task.Delay(10);
+
+        Assert.True(wasRunningDuringOperation);
+        tcs.SetResult(true);
+        await runTask;
+
+        Assert.False(vm.IsRunning);
+    }
+
+    [Fact]
+    public async Task ProgressValue_AndStatusMessage_UpdateViaIProgress()
+    {
+        var vm = new TestViewModel();
+        vm.OperationFunc = async (ct, progress) =>
+        {
+            progress.Report(new OperationProgress(50, 100, "halfway"));
+            await Task.Yield();
+        };
+
+        await vm.RunCommand.ExecuteAsync(null);
+
+        // Allow dispatcher to process
+        await Task.Delay(20);
+
+        Assert.Equal(50, vm.ProgressValue);
+        Assert.Equal("halfway", vm.StatusMessage);
+    }
+
+    [Fact]
+    public async Task CancelCommand_DuringOperation_SetsStatusMessageToCancelled()
+    {
+        var vm = new TestViewModel();
+        var started = new TaskCompletionSource<bool>();
+
+        vm.OperationFunc = async (ct, p) =>
+        {
+            started.SetResult(true);
+            await Task.Delay(5000, ct); // Will be cancelled
+        };
+
+        var runTask = vm.RunCommand.ExecuteAsync(null);
+        await started.Task;
+
+        vm.CancelCommand.Execute(null);
+        await runTask;
+
+        Assert.Contains("cancel", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
+        Assert.False(vm.IsRunning);
+    }
+
+    [Fact]
+    public async Task OperationCanceledException_IsCaughtGracefully_IsRunningBecomesFalse()
+    {
+        var vm = new TestViewModel();
+        vm.OperationFunc = (ct, p) => throw new OperationCanceledException();
+
+        // Should not throw
+        await vm.RunCommand.ExecuteAsync(null);
+
+        Assert.False(vm.IsRunning);
+    }
+
+    [Fact]
+    public async Task ExceptionDuringOperation_SetsStatusMessageToErrorText_IsRunningBecomesFalse()
+    {
+        var vm = new TestViewModel();
+        vm.OperationFunc = (ct, p) => throw new InvalidOperationException("test error");
+
+        await vm.RunCommand.ExecuteAsync(null);
+
+        Assert.False(vm.IsRunning);
+        Assert.Contains("test error", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public async Task RunCommand_CannotBeInvoked_WhileIsRunning()
+    {
+        var vm = new TestViewModel();
+        var tcs = new TaskCompletionSource<bool>();
+
+        vm.OperationFunc = async (ct, p) => await tcs.Task;
+
+        var runTask = vm.RunCommand.ExecuteAsync(null);
+        await Task.Delay(10); // Let it start
+
+        Assert.False(vm.RunCommand.CanExecute(null));
+
+        tcs.SetResult(true);
+        await runTask;
+
+        Assert.True(vm.RunCommand.CanExecute(null));
+    }
 }