Allow empty API keys for custom OpenAI endpoints in the macOS app

[ainopara]

Background

This PR is specific to the macOS app under macOS/WritingTools. Other platform implementations live in separate codebases and are not affected.

I use LM Studio as a local LLM backend for the macOS app. LM Studio exposes an OpenAI-compatible API, so the macOS app's OpenAI-compatible provider can call it by setting a custom base URL such as a local localhost endpoint.

In this local-provider scenario, an API key is not required. Requiring the API key field to be non-empty prevents otherwise valid OpenAI-compatible local endpoints from working in the macOS app.

Summary

• Allow the macOS app's OpenAI-compatible provider to use custom base URLs without requiring an API key.
• Omit the Authorization header for custom OpenAI-compatible requests when the API key field is empty.
• Continue requiring an API key when the default OpenAI endpoint is used.

Testing

• Verified in a local signed macOS build with an LM Studio-compatible local endpoint and an empty API key field.
• Confirmed the combined local branch still builds successfully with xcodebuild -project macOS/WritingTools.xcodeproj -scheme WritingTools -configuration Release -destination 'platform=macOS'.