refactor(internal): extract doStream helpers (Commit 2.9)

- Create stream-usage-tracker.ts for usage accumulation
- Create stream-content-tracker.ts for text/reasoning delta handling
- Create stream-tool-call-handler.ts for tool call state management
- Add REASONING_ID and TEXT_ID constants
- Reduce doStream function from ~300 to ~120 lines (-60%)

Author: brandonkachen