refactor(ocpp): extract responseHandler and incomingRequestHandler into base classes

refactor(ocpp): extract responseHandler and incomingRequestHandler into base classes

Both methods were 95% identical across OCPP 1.6 and 2.0 subclasses
(~160 lines of pure duplication). Move the shared logic into the base
classes, parameterized by 3 abstract properties each subclass provides:
- bootNotificationRequestCommand / pendingStateBlockedCommands
- csmsName ('central system' vs 'CSMS')
- isRequestCommandSupported / isIncomingRequestCommandSupported

Zero behavior change — pure refactoring validated by 1853 passing tests.