- Enhanced saveUsageStats to include cache tokens (cache_read, cache_creation), reasoning tokens, and estimated flag
- Replaced logUsage calls in stream.js with saveUsageStats (2 locations)
- Removed duplicate saveUsageStats call from streamingHandler.js onStreamComplete
- Removed logUsage function and unused imports from usageTracking.js
- Each streaming request now writes exactly 1 usage record instead of 2