[TCGC] [Bug] TCGC should return usage as 0 for model when it is only used in protocol method's response body

[haolingdong-msft]

Issue

When a model is only used in protocol method's response body. TCGC returns the model usage as 256, not 0.
For example, for below case, ResourceD is only used in protocol method's response body, but its usage is 256. While ResourceC which is only used in protocol method's request body, its usage is 0. I think ResourceC's usage is correct, but ResourceD's usage is wrong.

  @doc("When set protocol true and convenient false, only the protocol method should be generated, ResourceC and ResourceD should not be generated")
  @route("onlyProtocol")
  @post
  @convenientAPI(false)
  @protocolAPI(true)
  onlyProtocol(@body body: ResourceC): ResourceD;

detailed test case

Solution

When a model is used in protocol method's response body, TCGC returns the usage as 0 no matter it's input or output.