feat: add support for openai models

[aayushpurplesectors]

Description

This PR adds support for integrating direct OpenAI models into Passmark. It follows the existing architectural patterns set by the direct Anthropic and Google integrations. Users can now pass models using the openai/ prefix (e.g., openai/gpt-4o) directly to configuration.

What Changed

• Imported and configured createOpenAI in src/models.ts.
• Added the _openai lazy singleton in src/models.ts with graceful missing-key error handling mapping to ConfigurationError.
• Updated the switch case in resolveModel() to correctly route "openai" string prefixes.
• Updated README.md to document the new OPENAI_API_KEY environment variable.
• Added an entry to CHANGELOG.md under the [Unreleased] section detailing the change.

Why It's Needed

Although users can use Vercel AI Gateway or OpenRouter for OpenAI routing, native support for @ai-sdk/openai enables developers bypassing those systems to utilize models like GPT-4o or o3-mini cleanly and directly using their OPENAI_API_KEY.

Checklist

• Fast passing npm run build locally
• No type errors or linting flags raised
• Documentation affected by the changes has been updated (README.md)
• Added CHANGELOG.md notes to the [Unreleased] section
• Simple, focused, and un-bloated

[panda-sandeep]

Hi @aayushpurplesectors sorry about the delay here, but I am reviewing it now. Do you mind resolving the merge conflict please?