Add .empty() method to response builder for no-body responses

[Copilot]

Summary

Adds an .empty() method to the response builder to explicitly signal that a response has no body. Generated handler stubs for operations with no response content now call .empty() instead of returning the bare response object.

// Before (generated for DELETE /pet/{petId} with no body)
export const DELETE: deletePet = async ($) => {
  return $.response[400];
};

// After
export const DELETE: deletePet = async ($) => {
  return $.response[400].empty();
};

Original Prompt

Add an .empty() method where applicable to make the fact that a response has no body more explicit.

When generating code for a handler that has no response body, attach .empty().

Manual acceptance tests

• A route handler generated from an OpenAPI operation with no response body (e.g., DELETE /pet/{petId}) contains .empty() in the generated stub
• Calling .empty() on $.response[204] returns a response with the correct status and no body
• Calling .empty() after .header(...) preserves the headers
• A route with a response body (e.g., GET /pet/{petId}) is unchanged and still generates .random()
• TypeScript: .empty() is available on a builder for a response with no content; it is absent (type error) when content is defined

Tasks

• Added empty() to ResponseBuilder, WideResponseBuilder, and GenericResponseBuilderInner interfaces; typed as present only when [keyof Response["content"]] extends [never]
• Implemented empty() in createResponseBuilder as { ...this, content: undefined }
• Updated OperationCoder.write() to emit .empty() for no-body responses
• Updated snapshots for operation-coder and end-to-end generate tests
• Added unit and type-level tests
• Documented .empty() in docs/reference.md
• Resolved merge conflicts with main's counterfact-types refactor (one type per file with JSDoc comments); applied empty() changes to the new individual files (response-builder.ts, wide-response-builder.ts, generic-response-builder.ts)

[pmcelhaney]

@copilot address merge conflicts

[Copilot]

@copilot address merge conflicts

Merged main and resolved the conflicts (commit b386cf4). Main had refactored counterfact-types into one file per type — I applied the empty() changes to the new individual files (response-builder.ts, wide-response-builder.ts, generic-response-builder.ts) and preserved both my type tests and the new no-body response tests from main. All 43 test suites and type tests pass.