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.