fix(core): fail closed on RPD counter errors

[cursor]

📝 Description

What: fail closed when the bot or quiz Gemini RPD counter cannot be atomically updated in DynamoDB, and map that condition to existing provider-unavailable handling before any Gemini request is made.

Why: the previous ClientError path returned (0, True), which let production traffic bypass the daily RPD quota if DynamoDB throttled, IAM/table configuration broke, or the counter table was temporarily unavailable.

Verification:

• python3 -m compileall -q src tests/test_rate_limit_fail_closed.py
• python3 -m pytest tests/test_rate_limit_fail_closed.py (4 passed)

🚀 Type of Change

• 🐛 Bug Fix (Fix an issue)
• ✨ New Feature (Add a feature)
• 🏗️ Infrastructure (CDK/AWS resource changes)
• ♻️ Refactoring (Code refactoring, no logic changes)
• 📚 Documentation (Documentation update)

🏗️ Infrastructure Changes (Crucial)

• Resources Created: None
• Resources Modified: None
• Resources Deleted: None
• Cost Impact: Prevents quota/cost guard bypass during RPD counter failures.

✅ Checklist

• I have performed a self-review of my code.
• I have run pre-commit run --all-files locally and fixed all issues.
• I have added/updated Unit Tests (pytest).
• My changes do not generate new warnings.
• (If Infra change) I have run cdk diff locally to verify changes.

🔗 Related Issues

• Fixes refactor: Update bot and Gemini client for improved performance #71