refactor(ocpp): make buildRequestPayload the authoritative builder for all commands

refactor(ocpp): make buildRequestPayload the authoritative builder for all commands

buildRequestPayload now calls the centralized builders for
TRANSACTION_EVENT (via buildTransactionEvent) and STATUS_NOTIFICATION
(via buildStatusNotificationRequest) in both OCPP versions, matching
the existing pattern for START_TRANSACTION/STOP_TRANSACTION in 1.6.

Callers pass minimal params (eventType, connectorId, status, etc.)
and buildRequestPayload constructs the complete spec-compliant payload.

sendTransactionEvent's offline path builds directly for queueing since
the queue stores pre-built payloads sent as-is on reconnect.