Fix: MongoServerError: bad auth / MongoNetworkError: connect ECONNREFUSED / MongooseServerSelectionError
How to fix MongoDB 'MongoServerError: bad auth Authentication failed', 'MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017', and 'MongooseServerSelectionError' connection errors. Covers MongoDB not running, connection string format, Atlas network access, Docker networking, authentication, DNS/SRV issues, TLS/SSL, and Mongoose options.