Delivered: Response struct now carries the originating provider atom for downstream cost tracking.

Key accomplishments:
- Response.provider field (atom | nil) with backward-compatible nil default
- All 4 adapters (OpenAI, Anthropic, OpenRouter, TestProvider) set provider in parse_response/1
- Fix race condition in TestProvider.stop_state/1 on_exit cleanup
- 422 tests passing, 0 regressions