Unhelpful error when response codes are integers

[SJrX]

I just spent about an hour trying to figure out what was wrong with the a random spec, I got a cryptic error:
SchemaError: got null, want object (at )

Here is a test case that shows the issue:

package main

import (
	"fmt"
	"testing"

	"github.com/pb33f/libopenapi"
	validator "github.com/pb33f/libopenapi-validator"
)

const specWithIntegerResponseCode = `openapi: 3.1.0
info:
  title: Test
  version: 1.0.0
paths:
  /test:
    get:
      responses:
        200:
          description: OK
`

func TestIntegerResponseCodeProducesUnhelpfulError(t *testing.T) {
	doc, err := libopenapi.NewDocument([]byte(specWithIntegerResponseCode))
	if err != nil {
		t.Fatalf("Failed to parse document: %v", err)
	}

	v, errs := validator.NewValidator(doc)
	if len(errs) > 0 {
		t.Fatalf("Failed to create validator: %v", errs)
	}

	valid, validationErrs := v.ValidateDocument()

	fmt.Println("=== Integer response code (200:) ===")
	fmt.Printf("Valid: %v\n", valid)
	if !valid {
		fmt.Println("Validation errors:")
		for i, e := range validationErrs {
			fmt.Printf("  [%d] Message: %s\n", i, e.Message)
			fmt.Printf("      Reason: %s\n", e.Reason)
			fmt.Printf("      HowToFix: %s\n", e.HowToFix)
			if len(e.SchemaValidationErrors) > 0 {
				for _, sve := range e.SchemaValidationErrors {
					fmt.Printf("      SchemaError: %s (at %s)\n", sve.Reason, sve.Location)
				}
			}
		}
	}

	// The spec is technically invalid (integer instead of string), so validation
	// should fail. But the error message should be helpful.
	if valid {
		t.Error("Expected validation to fail for integer response code")
	}

	// Check that we get the unhelpful error
	if len(validationErrs) > 0 && len(validationErrs[0].SchemaValidationErrors) > 0 {
		reason := validationErrs[0].SchemaValidationErrors[0].Reason
		if reason == "got null, want object" {
			t.Fatalf("BUG CONFIRMED: Error message is 'got null, want object' instead of something helpful like 'response code must be a string'")
		}
	}
}

[SJrX]

Sorry that code was generated by Claude, I understand that the spec is indeed invalid, it was just very hard to figure out why.