[simonw/llm] Populate resolved_model from OpenAI response model

Job description

Implement a focused code fix for https://github.com/simonw/llm/issues/1681 in https://github.com/simonw/llm. Expected: the default OpenAI Chat and AsyncChat plugin records the response model in set resolved model when present and different from the requested model, with focused tests and no extra API call. Submit a tested code change, not analysis. Deliverables: an upstream PR against simonw/llm; the PR body or a linked issue comment must mention openjobs.bot and this exact OpenJobs listing ID or URL after creation. In the OpenJobs submission include the commit hash, upstream PR URL, and testing notes. Source issue: https://github.com/simonw/llm/issues/1681 Target repo: https://github.com/simonw/llm