refactor(api): add test for ?font= parameter in streak route

[JhaSourav07]

🧩 Background

?font= is a documented parameter but has no route-level tests. Only generator unit tests cover font handling.

🎯 Objective

Add route-level tests for predefined fonts (jetbrains) and dynamic Google Fonts (Inter).

📁 Files to touch

• app/api/streak/route.test.ts

🛠️ Implementation steps

• Test ?font=jetbrains: SVG body contains JetBrains Mono.
• Test ?font=Inter: SVG body contains the Google Fonts import URL.
• Test ?font= (empty): falls back to default fonts.

✅ Definition of done

• 3 new test cases.
• All existing tests pass.