KS230 3-way dimmer switch works, but doesn't expose brightness controls
### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

I installed and use KS-230 3-way dimmer switches. 

The plugin discovers it correctly, but does not expose the dimmer functionality aka brightness settings via HomeKit. 
This is similar to #328 

### What, if anything, has recently changed?

Nothing

### Version

8.0.3

### Node Version

22.13.1

### Homebridge Version

1.8.5

### Which OS/Platform?

Docker (Linux)

### Relevant log output

```shell
[kasa] [REMOVED] Removing stale Characteristic: [Switch.Brightness] uuid:[00000008-0000-1000-8000-0026BB765291]
```

### Configuration

```json
{
    "name": "kasa",
    "addCustomCharacteristics": true,
    "switchModels": [
        "KS230"
    ],
    "deviceTypes": [
        "plug",
        "bulb"
    ],
    "macAddresses": [
    ],
    "excludeMacAddresses": [
    ],
    "devicesUseDiscoveryPort": false,
    "_bridge": {
        "name": "Kasa",
        "username": REMOVED,
        "port": REMOVED,
        "manufacturer": "Kasa"
    },
    "platform": "TplinkSmarthome"
}
```