[specs] add test for discriminated models with union discriminator

[iscai-msft]

Please add a test for a scenario like this

@service
namespace Test;

@discriminator("type")
model ResponseFormat {
  type: "text" | "json_object" | "json_schema";
}

model ResponseFormatText extends ResponseFormat {
  type: "text";
}

op foo(): ResponseFormatText;

You can choose clearer names and test more scenarios