feat(validation): implement Rule 47 for response content-type and schema

[Junnygram]

Closes #719. Ensures 2xx responses (except 204) have a content block and schema.

[gemini-code-assist]

Code Review

This pull request introduces Rule 47 to the API audit validation, which mandates that all 2xx responses, excluding 204 (No Content), must include a content block and a defined schema. The implementation includes the validation logic in rules_design.go and corresponding unit tests. Feedback suggests extending the exclusion list to include the 205 (Reset Content) status code, as per RFC 9110, which also prohibits a response body. This involves updating the rule's logic, documentation, violation messages, and adding a specific test case for the 205 status code.

[gemini-code-assist]

Rule 47 should also exclude the 205 Reset Content status code. According to RFC 9110, a 205 response MUST NOT include a content part, similar to 204. I've updated the comment to reflect this.

Suggested change

	// --- Rule 47: 2xx responses (except 204) must have content and schema ---
	// --- Rule 47: 2xx responses (except 204 and 205) must have content and schema ---

[gemini-code-assist]

The 205 Reset Content status code is defined by RFC 9110 as a response that must not contain a body. It should be excluded from this validation rule to prevent false positives when auditing valid API specifications.

Suggested change

	if !strings.HasPrefix(code, "2") || code == "204" {
	if !strings.HasPrefix(code, "2") || code == "204" || code == "205" {

[gemini-code-assist]

Updating the violation message to include 205 in the list of excluded status codes for clarity.

Suggested change

	Message: fmt.Sprintf("%s — response %s is missing a `content` block. All 2xx responses (except 204) must declare a content-type.", label, code),
	Message: fmt.Sprintf("%s — response %s is missing a content block. All 2xx responses (except 204 and 205) must declare a content-type.", label, code),

[gemini-code-assist]

Adding a test case for the 205 Reset Content status code to ensure it is correctly handled as an exception to Rule 47.

}

func TestCheckRule47_Valid205(t *testing.T) {
	doc := &openapi3.T{
		OpenAPI: "3.0.0",
		Info:    &openapi3.Info{Title: "Test", Version: "v1"},
		Paths:   openapi3.NewPaths(),
	}
	responses := openapi3.NewResponses()
	desc := "Reset Content"
	responses.Set("205", &openapi3.ResponseRef{Value: &openapi3.Response{Description: &desc}})
	doc.Paths.Set("/api/test", &openapi3.PathItem{
		Post: &openapi3.Operation{Responses: responses},
	})

	vs := checkRule47("api.yml", doc, AuditOptions{})
	if len(vs) != 0 {
		t.Errorf("expected 0 violations for 205 response without content, got %d", len(vs))
	}
}