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

[JhaSourav07]

🧩 Background

?size=small, ?size=large are documented URL parameters with no test coverage in app/api/streak/route.test.ts.

🎯 Objective

Add tests verifying that ?size=small produces a 400×280 SVG and ?size=large produces an 800×560 SVG.

📁 Files to touch

• app/api/streak/route.test.ts

🛠️ Implementation steps

• Assert width="400" and height="280" appear in the small SVG body.
• Assert width="800" and height="560" appear in the large SVG body.
• Assert medium (default) produces width="600".

✅ Definition of done

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