fix: return 400 for invalid JSON request bodies

[Omar-Tnt04]

Summary

Return 400 Bad Request instead of 404 Not Found when POST/PUT/PATCH requests receive an invalid non-object JSON body.

What changed

• Updated withBody in src/app.ts
• Updated withIdAndBody in src/app.ts
• Added tests in src/app.test.ts for invalid request bodies

Why

Previously, invalid bodies such as arrays, strings, or null would fall through to the final handler and return 404 Not Found, which is misleading.

A malformed request body should return 400 Bad Request.

Tests

All tests pass, including new coverage for:

• POST with array body → 400
• PATCH with string body → 400
• PUT with null body → 400