feat: implement TxEnded meter value accumulator per OCPP 2.0.1 spec §2.1

feat: implement TxEnded meter value accumulator per OCPP 2.0.1 spec §2.1

Per E06.FR.11 and J02.FR.10, TransactionEvent(Ended) must contain meter
values sampled every TxEndedInterval from transaction start, with the
final sample marked as Transaction.End context.

- Add TxEndedInterval to OCPP20RequiredVariableName enum
- Add start/stopEndedMeterValues and getTxEndedInterval
- Accumulate periodic samples during transaction, flush in Ended event
- Harmonize Updated/Ended naming across API, variables, and log messages
- Exclude transactionEndedMeterValues from serialized configuration
- Fix stopEndedMeterValues incorrectly called in startUpdatedMeterValues
- Restore JSDoc-free ConnectorStatus fields
- Fix DEFAULT_WS_PING_INTERVAL constant reference
- Update test fixtures and assertions for serialization coverage