Simplify C# model cancellation API

Use a defaulted cancellation token parameter on IModel.DownloadAsync now that custom implementers do not need to be preserved.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bmehta001

sdk/cs/src/Detail/Model.cs

@@ -78,14 +78,10 @@ public async Task<string> GetPathAsync(CancellationToken? ct = null)
         return await SelectedVariant.GetPathAsync(ct).ConfigureAwait(false);
     }
 
-    public async Task DownloadAsync(Action<float>? downloadProgress = null)
+    public async Task DownloadAsync(Action<float>? downloadProgress = null,
+                                    CancellationToken? ct = null)
     {
-        await SelectedVariant.DownloadAsync(downloadProgress).ConfigureAwait(false);
-    }
-
-    public async Task DownloadAsync(Action<float>? downloadProgress, CancellationToken ct)
-    {
-        await ModelExtensions.DownloadAsync(SelectedVariant, downloadProgress, ct).ConfigureAwait(false);
+        await SelectedVariant.DownloadAsync(downloadProgress, ct).ConfigureAwait(false);
     }
 
     public async Task LoadAsync(CancellationToken? ct = null)

sdk/cs/src/Detail/ModelVariant.cs

@@ -61,12 +61,8 @@ public async Task<string> GetPathAsync(CancellationToken? ct = null)
                                                     .ConfigureAwait(false);
     }
 
-    public async Task DownloadAsync(Action<float>? downloadProgress = null)
-    {
-        await DownloadAsync(downloadProgress, CancellationToken.None).ConfigureAwait(false);
-    }
-
-    public async Task DownloadAsync(Action<float>? downloadProgress, CancellationToken ct)
+    public async Task DownloadAsync(Action<float>? downloadProgress = null,
+                                    CancellationToken? ct = null)
     {
         await Utils.CallWithExceptionHandling(() => DownloadImplAsync(downloadProgress, ct),
                                                $"Error downloading model {Id}", _logger)

sdk/cs/src/IModel.cs

@@ -27,7 +27,9 @@ public interface IModel
     /// <param name="downloadProgress">
     /// Optional progress callback for download progress.
     /// Percentage download (0 - 100.0) is reported.</param>
-    Task DownloadAsync(Action<float>? downloadProgress = null);
+    /// <param name="ct">Optional cancellation token.</param>
+    Task DownloadAsync(Action<float>? downloadProgress = null,
+                       CancellationToken? ct = null);
 
     /// <summary>
     /// Gets the model path if cached.