fix: resolve flaky DownloaderTest hangs by flushing response streams

Fixes issue where DownloadWithXOriginalContentLength and
DownloadWithMismatchedContentLength tests would hang due to
HTTP response streams not being properly flushed after writing data.

The tests were causing CI failures with 2-minute timeouts because
the HTTP client would wait indefinitely for more data when the
response stream wasn't explicitly flushed.

Co-authored-by: ibetitsmike <[email protected]>

Author: blink-so[bot]

Tests.Vpn.Service/DownloaderTest.cs

@@ -323,6 +323,7 @@ public async Task DownloadWithXOriginalContentLength(CancellationToken ct)
             ctx.Response.ContentType = "text/plain";
             // Don't set Content-Length.
             await ctx.Response.OutputStream.WriteAsync("test"u8.ToArray(), ct);
+            await ctx.Response.OutputStream.FlushAsync(ct);
         });
         var url = new Uri(httpServer.BaseUrl + "/test");
         var destPath = Path.Combine(_tempDir, "test");
@@ -345,6 +346,7 @@ public async Task DownloadWithMismatchedContentLength(CancellationToken ct)
             ctx.Response.Headers.Add("X-Original-Content-Length", "5"); // incorrect
             ctx.Response.ContentType = "text/plain";
             await ctx.Response.OutputStream.WriteAsync("test"u8.ToArray(), ct);
+            await ctx.Response.OutputStream.FlushAsync(ct);
         });
         var url = new Uri(httpServer.BaseUrl + "/test");
         var destPath = Path.Combine(_tempDir, "test");