Fix CanCancelContentUploadHttps flakiness (Azure#21849)

Author: pakrym

sdk/core/Azure.Core/src/RequestContent.cs

@@ -142,10 +142,10 @@ public override void WriteTo(Stream stream, CancellationToken cancellationToken)
                 {
                     while (true)
                     {
-                        cancellationToken.ThrowIfCancellationRequested();
+                        CancellationHelper.ThrowIfCancellationRequested(cancellationToken);
                         var read = _stream.Read(buffer, 0, buffer.Length);
                         if (read == 0) { break; }
-                        cancellationToken.ThrowIfCancellationRequested();
+                        CancellationHelper.ThrowIfCancellationRequested(cancellationToken);
                         stream.Write(buffer, 0, read);
                     }
                 }

sdk/core/Azure.Core/tests/HttpPipelineRequestContentTests.cs

@@ -6,6 +6,7 @@
 using System.Text;
 using System.Text.Json;
 using System.Threading;
+using System.Threading.Tasks;
 using NUnit.Framework;
 
 namespace Azure.Core.Tests
@@ -21,7 +22,7 @@ public void StreamCopyToCancellation()
             var content = RequestContent.Create(source);
             var cancellation = new CancellationTokenSource();
             cancellation.Cancel();
-            Assert.Throws<OperationCanceledException>(() => { content.WriteTo(destination, cancellation.Token); });
+            Assert.Throws<TaskCanceledException>(() => { content.WriteTo(destination, cancellation.Token); });
         }
 
         [Test]