Fix XunitLoggerProvider crash when background work logs after test disposal (#1229)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/Common/Utils/XunitLoggerProvider.cs

@@ -34,7 +34,20 @@ public void Log<TState>(
                 sb.Append(exception.ToString());
             }
 
-            output.WriteLine(sb.ToString());
+            try
+            {
+                output.WriteLine(sb.ToString());
+            }
+            catch (InvalidOperationException)
+            {
+                // Ignore exceptions from xUnit's TestOutputHelper when the test has already completed.
+                // Background work may continue logging after xUnit has disposed the test context.
+            }
+            catch (NullReferenceException)
+            {
+                // xUnit v3 may throw NullReferenceException in TestOutputHelper.QueueTestOutput()
+                // when the internal queue has been torn down after test completion.
+            }
         }
 
         public bool IsEnabled(LogLevel logLevel) => true;