Fix generateImage edit mode content type error by using global.fetch for FormData

[Copilot]

Fixes #1913 where generateImage in edit mode was throwing:

Unsupported content type: 'text/plain'. This API method only accepts 'multipart/form-data' requests, but you specified the header 'Content-Type: text/plain'.

Problem

The OpenAIImageGeneration function was using the createFetch wrapper (cross-fetch) for all requests, including FormData requests in edit mode. Cross-fetch doesn't properly handle FormData content types and defaults to text/plain instead of letting FormData set the proper multipart/form-data boundary.

Solution

Modified the image generation to conditionally use global.fetch for multipart requests (edit mode with FormData) and the createFetch wrapper for JSON requests (generate mode):

// TODO: switch back to cross-fetch in the future  
const res = isMultipart ? await global.fetch(url, freq as any) : await fetch(url, freq as any);

This follows the same pattern already used in other parts of the codebase (WhisperASR transcription, speech generation) where FormData requires proper handling.

Testing

• Enhanced test/image-generation.test.ts with new test cases for both edit and generate modes
• All existing image generation tests continue to pass (17/17)
• Core packages build successfully with no breaking changes

The fix is minimal and surgical, maintaining backward compatibility while resolving the regression that broke image editing functionality.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• cdn.sheetjs.com
  • Triggering command: node /home/REDACTED/.local/share/pnpm/.tools/pnpm/10.12.3/bin/pnpm install (dns block)
• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)
• example.com
  • Triggering command: node (vitest 3) (dns block)
• https://api.github.com/markdown
  • Triggering command: node (vitest 3) (http block)
• https://api.github.com/repos/microsoft/genaiscript/branches
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• https://api.github.com/repos/microsoft/genaiscript/contents/README.md
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• https://api.github.com/repos/microsoft/genaiscript/contents/packages%2Fcore%2Fsrc
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• https://api.github.com/repos/microsoft/genaiscript/git/blobs
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• https://api.github.com/repos/microsoft/genaiscript/git/ref/heads%2Ftest-ignore
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• https://api.github.com/repos/microsoft/genaiscript/issues
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• https://api.github.com/repos/microsoft/genaiscript/labels
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• https://api.github.com/repos/microsoft/genaiscript/pulls
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• https://api.github.com/repos/microsoft/genaiscript/pulls/999999
  • Triggering command: node (vitest 2) (http block)
• https://api.github.com/test
  • Triggering command: node (vitest 3) (http block)
• microsoft.github.io
  • Triggering command: node (vitest 1) (dns block)
  • Triggering command: node (vitest 3) (dns block)
• telemetry.astro.build

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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