Update API Models for Tool Use

Before we can implement the logic, we need to update our API contract in `artisan_engine/models.py` to support the necessary data structures for tools, as defined by the OpenAI API.

**Acceptance Criteria:**

- Create Pydantic models for Tool, Function, ToolCall, etc.
- Update `OpenAIChatRequest` to accept a tools: List[Tool] and a tool_choice parameter.
- Update `OpenAIMessage` to handle a `tool_calls: List[ToolCall]` attribute for assistant messages and a `tool_call_id` for tool messages.

The work is in artisan_engine/models.py. Start by reading the existing OpenAIChatRequest and OpenAIMessage models. Look at the OpenAI API documentation for the Tool, Function, and ToolCall schemas. Create corresponding Pydantic models, then integrate them into the request and message models. Verify by checking that the updated models align with the OpenAI spec.

Vom Indexierungsmodell aus dem Issue-Text verfasst.