fix(test): replace Progress\<T\> with SyncProgress\<T\> in OssDownloadExecutorTests to avoid Linux CI flaky failure

System.Progress<T> posts to SynchronizationContext, which may defer/defer
callbacks on headless Ubuntu CI runners with .NET 10, causing progressValues
to be empty at assertion time. SyncProgress<T> invokes the handler synchronously,
eliminating the race.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JusterZhu

tests/CoreTest/Download/Executors/OssDownloadExecutorTests.cs

@@ -147,8 +147,10 @@ public async Task ExecuteAsync_WithProgress_Reports100Percent()
         var asset = CreateAsset();
         var destPath = Path.Combine(Path.GetTempPath(), $"test_{Guid.NewGuid()}.zip");
 
+        // Use a synchronous IProgress<T> instead of System.Progress<T>
+        // to avoid SynchronizationContext dispatch races on Linux CI runners.
         var progressValues = new List<DownloadProgress>();
-        var progress = new Progress<DownloadProgress>(p => progressValues.Add(p));
+        var progress = new SyncProgress<DownloadProgress>(p => progressValues.Add(p));
 
         try
         {
@@ -289,3 +291,16 @@ public async Task ExecuteAsync_NullProgress_DoesNotThrow()
 
     #endregion
 }
+
+/// <summary>
+/// Synchronous <see cref="IProgress{T}"/> that invokes the handler immediately.
+/// Unlike <see cref="System.Progress{T}"/>, this does not post to
+/// <see cref="SynchronizationContext"/>, making it deterministic in test environments
+/// where a sync context may defer or drop callbacks (e.g. headless Linux CI runners).
+/// </summary>
+internal sealed class SyncProgress<T> : IProgress<T>
+{
+    private readonly Action<T> _handler;
+    public SyncProgress(Action<T> handler) => _handler = handler ?? throw new ArgumentNullException(nameof(handler));
+    public void Report(T value) => _handler(value);
+}