fix(ocpp2): revert UI transaction handlers to simple passthrough

fix(ocpp2): revert UI transaction handlers to simple passthrough

Remove handleUIStartTransaction and handleUIStopTransaction which
duplicated connector state management with incomplete initialization
(missing energy register, groupIdToken, StatusNotification, error
rollback) and bypassed authorization checks.

Restore handleTransactionEvent as a simple requestHandler passthrough,
consistent with all other broadcast channel command handlers.