test(apistreakpngroute): add mock integration tests

[imuniqueshiv]

Description

Fixes #6791

Adds mock integration tests for app/api/streak/png/route.ts to verify the PNG conversion pipeline while keeping all external dependencies isolated.

What was tested?

• Verifies requests are delegated to the upstream SVG route.
• Verifies SVG content returned by the upstream route is passed to Resvg.
• Verifies Resvg is initialized with the expected rendering configuration.
• Verifies the complete SVG → PNG conversion pipeline executes successfully.
• Verifies cache headers from the upstream SVG response are preserved in the PNG response.

Pillar

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

Visual Preview

N/A (test-only change)

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 and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have started the repo.
• I have made sure that i have only one commit to merge in this PR.

[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3917.41 KB	3917.41 KB	0 B
Total CSS	325.86 KB	325.86 KB	0 B

[Aamod007]

Great work on these PNG route integration tests! I reviewed \�pp/api/streak/png/route.mock-integrations.test.ts\ and successfully mocking @resvg/resvg-js\ to assert the rendering pipeline and cache header forwarding is impressive. This ensures the buffer conversion works smoothly without needing actual image rendering in CI. All tests are passing successfully!

Labels assigned:

• \level:intermediate: Mocking third-party native libraries like
  esvg\ and verifying buffer pipelines requires solid testing knowledge.
• \quality:clean: Clean \�i.hoisted\ mocks and well-structured tests.
• \ ype:testing: Purely focused on integration tests.