* Change default health check interval from 30s to 15s.
* Always start background health checks regardless of auto-reconnect setting.
* Add tests for health checker functionality and default configurations.
* Introduced NewConnectionFromDB function to create connections from existing *sql.DB instances.
* Added ExistingDBProvider to wrap existing database connections for dbmanager features.
* Implemented tests for NewConnectionFromDB and ExistingDBProvider functionalities.
Fixes deadlock caused by calling public methods (Bun, GORM, Native)
while holding a write lock. The public methods attempt to acquire
their own locks, causing the goroutine to freeze.
Solution: Call provider methods directly within the adapter getters
since they already hold the necessary write lock.
