bug(Overlay): how can i set default panelClass to cdkConnectedOverlay

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

19.x

### Description

### v19
In Angular CDK v19, I rewrote the overlay behavior to automatically apply different built-in panelClass values depending on the context. This allowed my custom default panelClass to coexist with user-provided panelClass values without being overridden.
```
@Injectable()
export class TestOverlay extends Overlay {
private getOverlayPanelClasses(config: OverlayConfig) {
let classes = ['test']; //'test' is only for examples
if (config.panelClass) {
if (Array.isArray(config.panelClass)) {
classes = classes.concat(config.panelClass);
} else {
classes.push(config.panelClass as string);
}
}
return classes;
}

create(config?: OverlayConfig): OverlayRef {
const overlayConfig: OverlayConfig = {
...config,
panelClass: this.getOverlayPanelClasses(config)
};
return super.create(overlayConfig);
}
}
```

### v21
After upgrading to Angular CDK v21 and switching to injection via the `CDK_CONNECTED_OVERLAY_DEFAULT_CONFIG` token, I found that whenever a component provides `cdkConnectedOverlayPanelClass` , the default panelClass I set through the default config gets completely overridden.

```
{
provide: CDK_CONNECTED_OVERLAY_DEFAULT_CONFIG,
useFactory: () => {
return {
panelClass: 'test',
};
}
}
```

There are many places in my project where cdkConnectedOverlayPanelClass is used. What should I change now?

### Reproduction

StackBlitz link:
Steps to reproduce:
1.
2.

### Expected Behavior

Provide a way to set a default class for panelClass.

### Actual Behavior

Setting CDK_CONNECTED_OVERLAY_DEFAULT_CONFIG is completely overridden by the custom input.

### Environment

- Angular: 21
- CDK/Material: 21
- Browser(s): any
- Operating System (e.g. Windows, macOS, Ubuntu):

Start by tracing how CDK_CONNECTED_OVERLAY_DEFAULT_CONFIG and the cdkConnectedOverlayPanelClass input are combined in the connected overlay implementation. Compare the v19 custom Overlay.create behavior with the v21 default-config path, then identify a reproducible test case showing whether the default and input panel classes should be merged.

Written by the indexing model from the issue text.