refactor: audit-driven improvements — use BaseError, fix barrel bypass, remove redund...

refactor: audit-driven improvements — use BaseError, fix barrel bypass, remove redundant console

- Replace throw new Error with BaseError in OCPP20CertificateManager and
  OCPP20VariableManager (3 occurrences)
- Move getMessageTypeString from OCPPServiceUtils to Helpers, resolving
  the barrel bypass in ErrorUtils — consumers import via charging-station
  barrel, tests moved to Helpers.test.ts
- Remove redundant console.warn in Helpers (logger.warn already called)
- Remove unused chalk import from Helpers