Simplify ai_streaming by delegating to ai_mode_streaming.
Extract sequence coordination into dedicated module.
Add worker pool management for concurrent AI task handling.
Refine ai_react_streaming for better delta chunk handling.
Billing is now handled internally by chat_service.process via record_ai_session.
Remove the old billing.rs file and explicit record_ai_usage calls from all 4
AI streaming modes (nonstreaming, react_nonstreaming, react_streaming, streaming).
- ai_react_nonstreaming now passes real input/output tokens to billing
- Was passing hardcoded 0,0 despite destructuring token data
- Also fix unused variable warnings
- process_react now returns (String, i64, i64) tuple with token counts
- Extract token stats from rig Agent FinalResponse usage field
- Both streaming and non-streaming ReAct modes now bill correctly
- Save thinking_content as {"__chunks__": [{type, content}]} for replay
- Tool call sanitization — don't expose raw results to frontend
- Billing record_ai_usage integration
- Room service module refactoring into service/ directory
