Support model flatten from two layers of extends in TypeSpec

[HarveyLink]

Bug Description

The .NET code gen doesn't support model faltten with 2 layers of extends in TypeSpec.
Sample link: https://github.com/Azure/azure-rest-api-specs/blob/897f19441a4cc240eba138296f31e50841a6b57f/specification/edgeorder/resource-manager/Microsoft.EdgeOrder/EdgeOrder/models.tsp#L1648
e.g.

@@flattenProperty(ProductLine.properties);

model ProductLine {
  properties?: ProductLineProperties;
}
model ProductLineProperties extends CommonProperties {
  products?: Product[];
}
model CommonProperties extends BasicInformation {
  filterableProperties?: FilterableProperty[];
}
model BasicInformation {
  displayName?: string;
...
}

The properties from BasicInformation and CommonProperties will missing in the generated class.

Expected Behavior

All the properties from BasicInformation and CommonProperties models get generated in ProductLine class.

Actual Behavior

Only one property: products is generated.