feat: add DI support to consumeCreditsAndAddAgentStep and calculateUsageThisCycle

Added dependency injection support to:
- consumeCreditsAndAddAgentStep: DI for withSerializableTransaction, trackEvent, reportPurchasedCreditsToStripe
- calculateUsageThisCycle: DI for db

New test file: balance-calculator.test.ts with 11 tests covering:
- Database query injection
- Transaction handling with mocked dependencies
- Analytics tracking verification
- Stripe reporting verification
- BYOK user handling
- Multi-grant consumption
- Error handling for missing grants and failed inserts
- Latency calculation

All 185+ billing tests pass.

Author: brandonkachen