OpenAPI schema transformers are not called for base types

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

Create a program like this

```
[JsonDerivedType(typeof(B), "b")]
[JsonDerivedType(typeof(C), "c")]
public class A
{
public int I { get; set; }
}
public class B : A
{
public int J { get; set; }
}
public class C : A
{
public int K { get; set; }
}

class MySchemaTransformer : IOpenApiSchemaTransformer
{
public async Task TransformAsync(OpenApiSchema schema, OpenApiSchemaTransformerContext context, CancellationToken cancellationToken)
{
...
}
}

app.MapGet("/api/x", () =>TypedResults.Ok(new A());

builder.Services.AddOpenApi(options =>
{
options.AddSchemaTransformer();
}
```

The generated OpenAPI spec will contain 4 types: ABase (properites `i`), AB (properies `$type`, `i` and `j`), AC (properties `$type`, `i` and `k`) and A (`anyof(AB, AC, ABase)`)

The transformer is called for the types `A`, `AB` and `AC`, but *not* for `ABase`

### Expected Behavior

The schema transformer should be called for the (implicitly created) type `ABase`.

### Steps To Reproduce

_No response_

### Exceptions (if any)

_No response_

### .NET Version

10

### Anything else?

_No response_