Add preflight validation and cost estimation

[ericc59]

Summary

• Validates config, API keys, data files, and output directory before batch runs
• Estimates costs upfront to prevent surprise bills
• Integrates into run_all.py with --skip-preflight and --cost-limit flags

Usage

# Standalone
python -m arc_agi_benchmarking.utils.preflight --config gpt-4o-2024-11-20 --data_dir data/evaluation

# With cost limit
python cli/run_all.py --config o1-high --cost-limit 50.00

Test plan

• 22 unit tests added and passing
• Tested against real evaluation dataset (120 tasks)