refactor(ocpp2): extract buildTransactionEvent as standalone function, remove dead...

refactor(ocpp2): extract buildTransactionEvent as standalone function, remove dead context code

Extract buildTransactionEvent from OCPP20ServiceUtils static method to
exported standalone function, consistent with buildStatusNotificationRequest
and buildMeterValue.

Remove unused context overload from buildTransactionEvent and
sendTransactionEvent — all production callers pass triggerReason directly.

Remove dead code: OCPP20TransactionContext interface (not in OCPP 2.0.1
specs), selectTriggerReason method, TransactionContextFixtures, and
associated tests.