fix(image): return 502 when upstream request fails

[ShakilUrRehman21]

Summary

Fixes #98569

When an upstream image request fails due to a network error, such as DNS resolution failure or connection refusal, /_next/image currently returns a 500 Internal Server Error.

This changes fetchExternalImage to convert non-timeout upstream fetch failures into an ImageError with HTTP 502 (Bad Gateway), indicating that the upstream request failed.

Timeout errors continue to return 504 as before.

Changes

• Return ImageError(502) for rejected upstream image fetches.
• Preserve the existing 504 response for upstream timeouts.
• Add a regression test covering rejected upstream fetch() requests.

Testing

• ./node_modules/.bin/jest test/unit/image-optimizer/fetch-external-image.test.ts --runInBand
  • 8 tests passed
• pnpm --filter next build
  • Passed