Generating of `[ObservableObject]` fails if interface is declared but not implemented

### Describe the bug

If a class implements an interface that itself inherits `INotifyPropertyChanged` or `INotifyPropertyChanging` but does not implement the interface methods, code-generation aborts.

Current tests only test diagnostics for an _implementation_ of `INotifyPropertyChanged` / `INotifyPropertyChanging`:
https://github.com/CommunityToolkit/dotnet/blob/7b53ae23dfc6a7fb12d0fc058b89b6e948f48448/tests/CommunityToolkit.Mvvm.SourceGenerators.UnitTests/Test_SourceGeneratorsDiagnostics.cs#L33-L37

### Regression

_No response_

### Steps to reproduce

```cs
interface IStuff : INotifyPropertyChanged {
// ...
}

[ObservableObject]
class B : BaseClass, IStuff {
// No implementation of INotifyPropertyChanged
// source generation
}
```

### Expected behavior

The implementation of `INotifyPropertyChanged` should be generated.

### Screenshots

_No response_

### IDE and version

VS 2022

### IDE version

_No response_

### Nuget packages

- [X] CommunityToolkit.Mvvm (aka MVVM Toolkit)

### Nuget package version(s)

8.2.2

### Additional context

Similar to #620 but slightly different scenario

### Help us help you

Yes, I'd like to be assigned to work on this item