Adds `AssistantMessage.responseModel` on the openai-completions path: surfaces the concrete `chunk.model` when it differs from the requested id (e.g. OpenRouter `auto` -> `anthropic/...`).
Adds `AssistantMessage.responseModel` on the openai-completions path: surfaces the concrete `chunk.model` when it differs from the requested id (e.g. OpenRouter `auto` -> `anthropic/...`).