fix(fibRoute): add explicit type for fibN to resolve ESLint type errors

[DeepakReddyG]

Changes

• Added fibN: number to ensure the fibonacci call in fibRoute.ts is type-safe, fixing @typescript-eslint/no-unsafe-assignment.

Testing

• Ran npm run lint to confirm no ESLint errors in fibRoute.ts after fixing fib.ts.
• Tested the endpoint with GET /5 (got fibonacci(5) is 8), GET /abc (got fibonacci(undefined) is undefined), and GET /-1 (got fibonacci(-1) is undefined) to verify it works.

Fixes #2