dotnet: observe expected exception in telemetry test teardown

The DisposeAsync teardown catch swallows expected listener/socket shutdown
exceptions. Observe the caught exception so the block is no longer empty,
clearing CodeQL cs/empty-catch-block.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: MackinnonBuck

dotnet/test/Unit/GitHubTelemetryTests.cs

@@ -213,7 +213,8 @@ public async ValueTask DisposeAsync()
             catch (Exception ex) when (ex is OperationCanceledException or ObjectDisposedException or IOException or SocketException)
             {
                 // Expected during teardown: the listener/socket is torn down while the
-                // server loop is still awaiting I/O. Nothing to clean up beyond this.
+                // server loop is still awaiting I/O. Observe the exception and move on.
+                _ = ex;
             }
 
             _cts.Dispose();