fix: scrub API keys from evaluation scripts for security

chore: remove hardcoded API keys and replace with environment variable loading

chore: switch default Gemini model to gemini-1.5-flash for better free-tier limits

chore: switch default Gemini evaluation model to gemini-3.1-flash-lite

fix: add smart rate limit handling parsing wait_time from 429 errors for Gemini eval

feat: add eval_gemini.py to evaluate Gemini models via API