Fix test failure from background logging after xUnit test disposal

Add try-catch in XunitLoggerProvider to gracefully handle exceptions when
logging after xUnit's TestOutputHelper has been disposed. This prevents
NullReferenceException when background work continues after test completion.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/Common/Utils/XunitLoggerProvider.cs

@@ -34,7 +34,15 @@ public void Log<TState>(
                 sb.Append(exception.ToString());
             }
 
-            output.WriteLine(sb.ToString());
+            try
+            {
+                output.WriteLine(sb.ToString());
+            }
+            catch
+            {
+                // Ignore exceptions from xUnit's TestOutputHelper when the test has already completed.
+                // Background work may continue logging after xUnit has disposed the test context.
+            }
         }
 
         public bool IsEnabled(LogLevel logLevel) => true;