Fix race in ConnectAndReceiveNotification_InMemoryServer test

- Go back to not processing messages until IMcpServer.RunAsync is called
- style: consistently use mcpServer from parameter rather than features

Failed test run: https://github.com/modelcontextprotocol/csharp-sdk/actions/runs/14299175970/job/40070612363?pr=225

Author: halter73

tests/ModelContextProtocol.Tests/SseIntegrationTests.cs

@@ -77,8 +77,7 @@ public async Task ConnectAndReceiveNotification_InMemoryServer()
             mcpServer.RegisterNotificationHandler("test/notification", async (notification, cancellationToken) =>
             {
                 Assert.Equal("Hello from client!", notification.Params?["message"]?.GetValue<string>());
-                var server = httpContext.Features.GetRequiredFeature<IMcpServer>();
-                await server.SendNotificationAsync("test/notification", new { message = "Hello from server!" }, cancellationToken: cancellationToken);
+                await mcpServer.SendNotificationAsync("test/notification", new { message = "Hello from server!" }, cancellationToken: cancellationToken);
             });
             return mcpServer.RunAsync(cancellationToken);
         });