feat(output-options): add default-response-error for unmatched responses - #2561

Fixes #1923.

Problem

A response parsing function generated by ClientWithResponses silently accepts a response that matches no declared case:

switch {
case strings.Contains(rsp.Header.Get("Content-Type"), "json") && rsp.StatusCode == 200:
	...
case strings.Contains(rsp.Header.Get("Content-Type"), "json") && rsp.StatusCode == 404:
	...
}

return response, nil

If the server returns 500 and the spec declares only 200 and 404, no case matches. The wrapper is returned with err == nil and its typed fields left at their zero value, so a caller cannot distinguish "unrecognised response" from "recognised response with an empty body". As raised in #1923, that makes an unexpected response easy to miss.

Change

New opt-in output option default-response-error, defaulting to false so existing generated output is unchanged. When enabled, the switch gains:

default:
	return nil, fmt.Errorf("unexpected response status: %d", rsp.StatusCode)

output-options:
  default-response-error: true

The error also propagates through <Op>WithResponse, which delegates to the parse function.

Interaction with a spec-declared default response

A spec that declares a default response still takes precedence: its case clause is emitted before the generated default: and is evaluated first. The new case only fires for responses the spec does not describe at all.

On the error value

It is a plain fmt.Errorf carrying the status code rather than a sentinel, to keep the change small and avoid emitting a new exported symbol into every generated package. If you would prefer an exported sentinel (for example ErrUnexpectedResponse) that callers can match with errors.Is, I am happy to switch to that.

Tests

New fixture internal/test/options/default_response_error/, following the existing response_getters layout: one spec declaring 200 + 404, generated twice into separate packages.

• enabled/: a 200 still parses normally, and a 500 returns an error naming the status code.
• default/: a 500 still returns the wrapper with no error, pinning the unchanged default behaviour.

make tidy && make test && make generate && make lint all pass locally, and make generate produces no changes outside the new fixture.

AI assistance was used to develop this change. I have reviewed it, tested it, and can explain every line.