fix(api): fail ai-generate provider errors

[cursor]

Summary

• Stop OpenAI, Gemini, and xAI ai-generate provider failures from being coerced into ok:true empty responses.
• Return non-2xx API errors for provider non-2xx or empty output so authenticated credit calls are refunded by the existing finalizer.
• Add a focused regression guard and wire it into npm test.

Validation

• node tests/ai-generate-provider-errors.test.mjs
• npm run build
• npm test