fix: do not report cable lock state for models that do not provide it

[nanomad]

Some models, such as the MG ZS EV, do not send ccuEleccLckCtrlDspCmd in the charge management data. The client library's charging_port_locked then returns False, so the gateway always published "unlocked", even with the cable locked (#271).

• The cable lock topic is published only when the car reports the field. Otherwise nothing is published and HA shows "unknown".
• After a successful lock or unlock command, the gateway publishes the commanded state. Cars that do report the lock overwrite it on the forced refresh.

Notes:

• On models that don't report the lock, the shown state is the commanded one. A manual lock or unlock is not reflected.
• A retained False published by older versions stays on the broker until the first lock or unlock command. It can be cleared with an empty retained publish on .../drivetrain/chargingCableLock.

Closes #271