refactor: apply production readiness recommendations including dataset caching, XSS protection, pure schemas, and JSON decoding logic.

feat: update inference, models, and server logic

final: submission ready

fix: update API_BASE_URL to router.huggingface.co, openenv validate passes

fix: update inference.py to mandatory [START][STEP][END] stdout format

fix: conform to OpenEnv base interface contract

PreferenceLab OpenEnv environment for RLHF preference simulation