refactor(ocpp): use union types in common code where possible

refactor(ocpp): use union types in common code where possible

- OCPPServiceUtils: use SampledValue union in generic constraints
  instead of inline OCPP16SampledValue | OCPP20SampledValue union;
  use StatusNotificationRequest union in satisfies check
- BroadcastChannel: use MeterValuesRequest/Response unions instead
  of OCPP20-specific types in version-dispatched handleMeterValues

Stack-specific types remain in version-switch branches where
TypeScript requires narrowed types for array push operations.