typespec1-4  Exercise 3: A 4xx Response for Create
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The lint rule operation-4xx-response, before and after adding a BadRequest (400) to create (books_v2.tsp adds the model below and returns Created<Book> | BadRequest | ApiError):

  @error
  model BadRequest {
    @statusCode _: 400;
    message: string;
  }

  == out_books
    30:7    warning  operation-4xx-response  Operation must have at least one `4XX` response.
    48:7    warning  operation-4xx-response  Operation must have at least one `4XX` response.
  ❌ Validation failed with 8 errors and 4 warnings.
  == out_books_v2
    30:7    warning  operation-4xx-response  Operation must have at least one `4XX` response.
  ❌ Validation failed with 8 errors and 3 warnings.

WHAT CHANGED
  Before: two operations had no 4XX response (list at line 30 of the generated file and create at line 48). After: only list.
  The 4XX warning for create went away because create now declares a 400. The generic 'default' response was already there and does not count as a 4XX for this rule.
  The 8 errors are unchanged: the missing summaries and security from chapters 2 and 5.
DO I AGREE THE REMAINING WARNING IS RIGHT?
  For list, only partly. A list that takes optional paging parameters can still be asked for something invalid (limit=-5), in which case a 400 would be reasonable, and a client library would like to know. If list has no parameters at all, a 4XX response would be invented. The linter cannot know; a person decides. Write the response if clients should handle it, and silence the rule for that operation otherwise.

WHY THIS WORKS AS AN ANSWER
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It shows a lint rule turning into a design question and a one-model change in the TypeSpec, with no edit to generated output. It also shows that a rule can be right for one operation and arguable for its neighbour.