refactor(ocpp2): centralize payload construction and eliminate duplication

refactor(ocpp2): centralize payload construction and eliminate duplication

Remove redundant timestamp injection from buildRequestPayload() for
StatusNotification and TransactionEvent — centralized builders already
provide timestamps and AJV schema validation catches any omission.

Extract buildStationInfoReportData() helper to deduplicate station info
report data construction between FullInventory and SummaryInventory in
buildReportData().

Normalize TriggerMessage Heartbeat to use OCPP20Constants.OCPP_RESPONSE_EMPTY
for consistency with buildRequestPayload().