Mark RunSessionHandler experimental (MCPEXP002) (#1383)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>
Co-authored-by: jeffhandley <1031940+jeffhandley@users.noreply.github.com>

samples/EverythingServer/Program.cs

@@ -51,6 +51,7 @@ builder.Services
     .WithHttpTransport(options =>
     {
         // Add a RunSessionHandler to remove all subscriptions for the session when it ends
+#pragma warning disable MCPEXP002 // RunSessionHandler is experimental
         options.RunSessionHandler = async (httpContext, mcpServer, token) =>
         {
             if (mcpServer.SessionId == null)
@@ -76,6 +77,7 @@ builder.Services
                 subscriptions.TryRemove(mcpServer.SessionId, out _);
             }
         };
+#pragma warning restore MCPEXP002
     })
     .WithTools<AddTool>()
     .WithTools<AnnotatedMessageTool>()