Introduces a new SANDBOX environment with a dedicated web app (`Mango.Sandbox.EndPoints`) for testing SignalR endpoints in isolation from the main application. Adds null plugin manager implementations to avoid plugin dependencies. Includes automated MSTest tests (`Mango.Sandbox.EndPoints.Tests`), a manual SignalR test client, and a PowerShell script for building and running the sandbox. Provides configuration files and health endpoints for robust, independent development and testing of FruitBank SignalR logic.
