[TCGC][Bug] TCGC SdkModelType.usage does not consider readonly case

[haolingdong-msft]

TSP:

  model ResultModel {
    name: string;
  }

  model RoundTripModel {
    @visibility("read")
    result: ResultModel;
  }

  @route("/modelInReadOnlyProperty")
  @put
  op modelInReadOnlyProperty(@body body: RoundTripModel): {
    @body body: RoundTripModel;
  };

detailed case can be found in this pr: Azure/cadl-ranch#587

In this case, TCGC returns UsageFlags for ResultModel is 6(UsageFlags.Input | UsageFlags.Output). But this is a readonly model, which should be output only, and usage should be 4(UsageFlags.Output).