fix: harden backend validation, parsing, and 404 handling

[ANIRUDDH-001]

Summary

Hardens b0bot backend stability by fixing critical issues in route validation, error handling, and LLM response parsing.

What Changed

• Routes: Added validation guards for missing/blank keyword parameters (returns 400)
• 404 Handler: Fixed response flow with explicit return statement
• Parser: Hardened toJSON() against None/empty/whitespace inputs and malformed lines

Why

Resolves Issue #161 and prevents IndexError crashes when users submit empty requests or the LLM returns malformed data.

Testing

• 5 route tests: keyword validation (missing/blank) + 404 responses
• 5 parser tests: None/empty/whitespace/malformed/single-item handling
• All 13 tests passing locally

Related

• Closes Improve backend stability by fixing LLM parsing, API validation, and 404 error handling #161
• Related to Improve backend stability in NewsService and NewsRoutes #162