- Add model field extraction in TokenUsage parsing for Claude, OpenAI, and Codex
- Prioritize response model over request model in usage logging
- Update model extractors to use parsed usage.model first
- Add tests for model extraction in stream and non-stream responses
Extract common request handling logic into dedicated modules:
- handler_config.rs: Usage parser configurations for each API type
- handler_context.rs: Request lifecycle context management
- response_processor.rs: Unified streaming/non-streaming response handling
Reduces handlers.rs from ~1130 lines to ~418 lines (-63%), eliminating
repeated initialization and response processing patterns across the
four API handlers (Claude, Codex Chat, Codex Responses, Gemini).
YoVinchen