fix(api): classify compare route GitHub errors

[eshaanag]

Description

Fixes #2204

Pillar

• 🛠️ Other (Bug fix, refactoring, docs)

What changed

The /api/compare route used Promise.allSettled, but both rejected fetch branches returned 404 unconditionally. That made GitHub rate limits and upstream failures look like missing users.

This PR adds a small helper that classifies rejected GitHub dashboard fetches before responding:

• not-found / could-not-resolve errors still return 404
• rate-limit style errors return 403 with the existing GitHub rate-limit message
• other upstream failures return 500 instead of being mislabeled as not-found

Tests were added for user1 rate-limit, user2 rate-limit, and a non-rate-limit upstream failure.

Visual Preview

N/A - JSON API status-code fix only.

Local verification

• npm run test -- app/api/compare/route.test.ts passed
• npm run format:check passed
• npm run lint passed with one existing warning in components/WallOfLove.tsx
• npm run typecheck passed
• npm run test passed: 83 files, 1438 passed, 1 expected fail
• npm run build fails locally with an opaque webpack error; this appears environment/upstream-local because CI build passed on the prior PR after the same local failure pattern

GSSoC 2026

This is raised and fixed under GSSoC 2026. Please add the relevant GSSoC/level labels if they do not sync from the issue automatically.

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• I have run npm run format / npm run format:check and npm run lint locally.
• I have run npm run test and all tests pass locally.
• My commits follow the Conventional Commits format.
• I have updated README.md if I added a new theme or URL parameter.
• I have starred the repo.
• I have made sure that I have only one commit to merge in this PR.
• The SVG output matches the CommitPulse quality standard.

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[eshaanag]

/addlabel bug api gssoc26

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