test(ocpp): strengthen mock call verification in OCPP 2.0 handler tests

test(ocpp): strengthen mock call verification in OCPP 2.0 handler tests

Replace loose assertions (assert.ok(>0), boolean flags) with strict
mock.fn() callCount() and argument verification across 4 test files:

- UnlockConnector: verify exact call count + STATUS_NOTIFICATION command
- ClearCache: replace 3 boolean flags with mock.fn() + callCount()
- CertificateSigned: verify closeWSConnection called/not-called per cert type
- ChangeAvailability: capture requestHandler via factory, verify side effects