refactor: code quality improvements

[ibrahimkizilarslan]

1. Removed duplicate validation (M3): The validate CLI command previously called LoadConfig (which inherently runs validation) and then called cfg.Validate() again. Removed the redundant call.
2. Unexported ActionHandlers map (M6): Made actionHandlers private to prevent global mutable state bugs. Added GetSupportedActions() and GetActionHandler() accessors to preserve testability.
3. Updated tests: Migrated all unit tests in pkg/engine and tests/engine to use the new action handlers accessor methods.
4. Constants for static data (M5): Changed allowPrivateNetworks from a global mutable variable to a constant to prevent accidental or malicious runtime modification.