fix(ocpp2): restore connector cleanup in TransactionEvent(Ended) response handler

fix(ocpp2): restore connector cleanup in TransactionEvent(Ended) response handler

Commit 710db15c removed cleanup from the Ended response handler assuming
callers always handle it. The UI broadcast channel passthrough bypasses
all caller-side cleanup, leaving connectors stuck in Occupied state.

Restore cleanupEndedTransaction in handleResponseTransactionEvent for the
Ended case, making the response handler the authoritative cleanup point
for all online paths. Add idempotency guard to cleanupEndedTransaction
so existing caller-side calls (needed for offline fallback) become no-ops
when the response handler already ran. Fix unconditional Available status
to respect ChangeAvailability(Inoperative) per G03.