feat(node): support OpenAI-compatible base URLs

[jimmyzhuu]

Summary

• Add optional credentials.baseUrl support for OpenAI-compatible chat completions endpoints in the Node OpenAI provider.
• Apply the custom endpoint to both OCR and extraction requests.
• Keep existing OpenAI usage unchanged when baseUrl is omitted.

Why

The Node SDK currently hardcodes https://api.openai.com/v1/chat/completions, which blocks OpenAI-compatible vision endpoints, local gateways, and compatible provider APIs.

This addresses the use cases raised in #161 and #171, and provides a complete, backward-compatible version of the base URL support attempted in #178.

Tests

• npx jest node-zerox/src/models/openAI.test.ts --runInBand
• npm run build
• Manual smoke path: run examples/node/qianfan.ts with QIANFAN_API_KEY set locally to verify an OpenAI-compatible vision endpoint can process the first page of the sample PDF and return non-empty Markdown.

Notes

baseUrl is optional, so existing OpenAI usage with only apiKey is unchanged.