Build errors when using CommunityToolkit.MVVM with Visual Studio extensions

### Describe the bug

I've asked around on SO and discord, but getting no joy. Forgive me if this is my misunderstanding. My SO post:
I'm getting build errors despite the SourceGenerator apparently working correctly. Example code:

```cs
namespace VisGit.ViewModels
{
public partial class MainViewModel: ObservableObject
{
[ObservableProperty]
string _exceptionMessageText = "Exception Text";

public MainViewModel()
{
ExceptionMessageText = "Test Change";
}
}
}
```
Intellisense shows the public property `ExceptionMessageText` as being generated successfully:

[![enter image description here][1]][1]

Also, source generator appears to work:

[![enter image description here][2]][2]

However, I get the build error `The name 'ExceptionMessageText' does not exist in the current context`:

[![enter image description here][3]][3]

Full code: https://github.com/stigzler/VisGit/blob/1-setup-base-extension-environment/VisGit/ViewModels/MainViewModel.cs

Not sure what I'm dong wrong. I'm using .net Framework 4.8 (is that the issue?). Any ideas?

[1]: https://i.sstatic.net/MBzU6OIpl.png
[2]: https://i.sstatic.net/M9oY3HpB.png
[3]: https://i.sstatic.net/DurdWT4E.png

### Regression

_No response_

### Steps to reproduce

```text
1. Create a new VSIX Project w/Tool Window project
2. Create any Class creating a new ObservableProperty via the Attribute
```

### Expected behavior

priavte var is converted to Public property and usable in same class. Project also builds without error "The name '[x]' does not exist in the current context"

### Screenshots

See OP

### IDE and version

VS 2022

### IDE version

4.8.09032

### Nuget packages

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

### Nuget package version(s)

8.2.2

### Additional context

_No response_

### Help us help you

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