fix(ocpp20): ocpp 2.0.1 conformance audit — 17 findings remediated

fix(ocpp20): ocpp 2.0.1 conformance audit — 17 findings remediated

- fix(types): widen DataTransfer data to JsonType for unconstrained payloads
- fix(types): narrow VariableAttributeType.type to AttributeEnumType
- fix(types): make ReportDataType.variableAttribute required per JSON schema
- fix(types): allow custom string values for UnitOfMeasure.unit
- fix(firmware): remove invalid AcceptedCanceled from FirmwareStatusEnumType
- fix(firmware): return AcceptedCanceled on cancel, trigger lifecycle for both
- fix(firmware): track lastFirmwareStatus for TriggerMessage (L01.FR.25/26)
- fix(firmware): accept UpdateFirmware with active transactions (L01.FR.06)
- fix(transaction): make evseId optional in RequestStartTransaction (F01.FR.10)
- fix(transaction): consult AuthorizeRemoteStart variable (F01.FR.02)
- fix(network): implement security profile downgrade detection (B09.FR.04)
- fix(network): implement slot-in-priority validation (B09.FR.05)
- fix(trigger): contextual EVSE validation for MeterValues/StatusNotification only
- fix(availability): save/restore pre-Inoperative connector statuses (G03.FR.07)
- fix(availability): add connector-level ChangeAvailability targeting