fix(files): report success cancellation and errors with safe next actions (U02)

tests/Duempelkas.Tests/FileOperationTests.cs

@@ -0,0 +1,41 @@
+using Duempelkas.App.Services;
+using Duempelkas.App.Services.Models;
+using Duempelkas.Infrastructure.Services;
+using Xunit;
+
+namespace Duempelkas.Tests;
+
+public class FileOperationTests
+{
+    [Fact]
+    public async Task Cancel_IsNeutral()
+    {
+        var service = new FileSaveService(new Dialog(null));
+        Assert.Equal(FileOperationStatus.Cancelled, (await service.SaveFileAsync([1], "test.pdf")).Status);
+    }
+
+    [Fact]
+    public async Task SaveAndFailure_AreDistinct_AndOpeningMissingFileIsError()
+    {
+        var folder = Path.Combine(Path.GetTempPath(), "duempelkas-files-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(folder);
+        try
+        {
+            var path = Path.Combine(folder, "Prüfung.pdf");
+            var service = new FileSaveService(new Dialog(path));
+            Assert.Equal(FileOperationStatus.Success, (await service.SaveFileAsync([1, 2, 3], "test.pdf")).Status);
+            Assert.Equal(new byte[] { 1, 2, 3 }, await File.ReadAllBytesAsync(path));
+            // A directory cannot be overwritten as a file; the existing file is preserved.
+            Assert.Equal(FileOperationStatus.Error, (await new FileSaveService(new Dialog(folder)).SaveFileAsync([4], "test.pdf")).Status);
+            Assert.Equal(FileOperationStatus.Error, (await service.OpenFileAsync(Path.Combine(folder, "missing.pdf"))).Status);
+            Assert.Equal(new byte[] { 1, 2, 3 }, await File.ReadAllBytesAsync(path));
+        }
+        finally { Directory.Delete(folder, true); }
+    }
+
+    private sealed class Dialog(string? path) : IFileDialogService
+    {
+        public Task<string?> SaveAsync(string suggestedFileName) => Task.FromResult(path);
+        public Task<string?> OpenBackupAsync() => Task.FromResult(path);
+    }
+}