test: cover getUpstashConfig missing-env and empty-string cases (#3031)

[aaniya22]

Summary

Adds test coverage for getUpstashConfig()'s two untested branches — both env vars set, and either env var being an empty string — closing the gap identified in the issue.

Closes #3031

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• Added getUpstashConfig returns { url, token } when both env vars are set to test/upstash-rest.test.mjs
• Added getUpstashConfig returns null when either env var is an empty string to test/upstash-rest.test.mjs
• No changes to src/lib/upstash-rest.ts itself — test-only PR

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How to Test

1. Checkout this branch
2. Run node --test test/upstash-rest.test.mjs
3. Confirm all 6 tests pass (4 pre-existing + 2 new)

Expected result: All tests pass, including the two new cases covering getUpstashConfig's previously-untested branches.

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Screenshots / Recordings

Not applicable — test-only change, no UI impact.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

Not applicable — no UI impact.

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Additional Context

Issue #3031 asked for a new test/upstash-rest.test.ts using Vitest, but a test/upstash-rest.test.mjs using Node's built-in node:test runner already exists and covers most of upstash-rest.ts (including upstashRateLimitFixedWindow and upstashTryAcquireLock). Rather than create a second, competing test file for the same module in a different framework, I extended the existing file with the two specific getUpstashConfig scenarios the issue called out that weren't yet covered. Happy to convert to a standalone Vitest file instead if that's preferred — just let me know and I'll follow up.

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GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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