After t-3446 (arena benchmarks) and t-3447 (drain integration) are built, write tests for:

• /api/models/arena/route.ts - POST handler, multi-model dispatch, cost estimation, error handling
• /api/models/drain - drain logic, benchmark-trigger-on-drain
• ModelArenaClient.tsx - prompt submission, result display, ranking
• AllowanceDrainPanel - toggle, trigger, benchmark integration
• Benchmark config persistence (loading/saving test suites)
• Arena results data model and ranking logic

Target: 80% coverage on arena and drain API routes. Document in TESTING-PLAYBOOK.md.