Covers plan section 4.1: pickChallenge 70/30 distribution, pickRankedChallenge never returns choices,
True/False auto-generation, special characters and long answers, all 16 challenge types produce valid
mock responses, and timeout/error answer verification. 113 → 125 tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>