Test: verify edge cases and empty inputs for compare API route

[diksha78dev]

Description

Fixes #4198

• Added comprehensive unit tests for app/api/compare/route.ts targeting edge cases, empty/missing inputs, and cached headers.
• Achieved complete test coverage of input validation, upstream error fallbacks, and caching edge cases.

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs, testing)

Visual Preview

N/A (Isolated Backend/API Route Tests)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (npx vitest run app/api/compare/route.empty-fallback.test.ts).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., test(ApiCompareRoute-empty-fallback): verify edge cases & empty inputs).
• I have updated README.md if I added a new theme or URL parameter. (N/A)
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts). (N/A)
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and faster PR support.

[github-actions]

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