[blazor] add missing namespace (#62529)

Author: pavelsavara

src/Components/Server/src/Circuits/CircuitActivitySource.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+namespace Microsoft.AspNetCore.Components.Server.Circuits;
+
 using System.Diagnostics;
 using Microsoft.AspNetCore.Components.Infrastructure.Server;