`[UIThreadTestMethod]` should have Analyzer to ensure parent class inherits from `VisualUITestBase`

### Describe the bug

The Source Generator tries to call `EnqueueAsync` on the class, but this is a method from `VisualUITestBase`, so if you try and use the generator attribute without inheriting from the class, you can an error about it not finding the method.

We should have an analyzer here to help guide folks to knowing that they have to inherit from the class. That and/or we should figure out a way to capture the dispatcher at the right point in test initialization so that we can just use it for the generated methods without needing a special base class?

### Steps to reproduce

```text
Use `UIThreadTestMethod` in a class not inheriting from `VisualUITestBase`
```

### Expected behavior

Help in guiding to the right path.

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [ ] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

Start by locating the source generator handling UIThreadTestMethod and the EnqueueAsync call, then inspect VisualUITestBase to confirm the required inheritance relationship. Done means the invalid attribute usage receives clear analyzer guidance, with tests covering a class that inherits from VisualUITestBase and one that does not.

Scritto dal modello di indicizzazione a partire dal testo della issue.