Add MySQL and MongoDB connectivity checks

[vidya381]

Extends the DB checks to cover MySQL and MongoDB. The detector already tracks both.

• MySQL → connect using MYSQL_URL, dependency: github.com/go-sql-driver/mysql
• MongoDB → connect using MONGODB_URI or MONGO_URL, dependency: go.mongodb.org/mongo-driver

Add internal/check/mysql.go and internal/check/mongo.go, register in registry.go under stack.MySQL and stack.MongoDB.