fix(http): honor OAS 3.1 schema dialect defaults

[cyphercodes]

Addresses #2797

Summary

Prism now falls back to the OpenAPI document's JSON Schema dialect when validating schema objects that do not declare their own $schema. For OpenAPI 3.1 documents, this selects the existing AJV 2020-12 instance by default so keywords such as unevaluatedProperties are enforced instead of silently falling through to draft-07.

Checklist

• The basics
  • I tested these changes manually in my local or dev environment
• Tests
  • Added or updated
  • N/A
• Event Tracking
  • I added event tracking and followed the event tracking guidelines
  • N/A
• Error Reporting
  • I reported errors and followed the error reporting guidelines
  • N/A

Additional context

Local verification:

• npm ci
• npm test -- packages/http/src/validator/validators/__tests__/utils.spec.ts --runInBand
• npm run build
• git diff --check HEAD^ HEAD