fix(api): add boundary and type validations for hint coordinates

[ankiitmishraa-tech]

Description

This PR resolves the issue where the /api/puzzle/hint endpoint accepts coordinates exceeding board dimensions and handles malformed/invalid inputs.

Changes Made

1. Upgraded Input Validation Schema:
   • Modified src/lib/validation/hint-validation.ts using strict refinements to reject non-numeric inputs, empty query values, floats, negative numbers, and NaN.
2. Added Boundary Checking:
   • Updated app/api/puzzle/hint/route.ts to ensure requested hint coordinates do not exceed the actual boardSize of the fetched puzzle.
3. Improved Error Status Handlers:
   • Wrapped database lookups inside a try-catch block to handle invalid puzzleId payloads gracefully, returning a 404 Not Found rather than a standard 500 Server Error.

Verification & Testing

Input	Expected	Status
puzzleId=1&x=10&y=10	400 Bad Request	✅
puzzleId=1&x=abc&y=3	400 Bad Request	✅
puzzleId=1&x=-2&y=3	400 Bad Request	✅
puzzleId=1&y=3 (x missing)	400 Bad Request	✅
puzzleId=9999&x=3&y=3	404 Not Found	✅
Closes #42

CC: @coder-zs-cse

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