Converter.items_to_messages cannot handle assistant message with both content and tool_calls

Currently the following item:

```python
{'role': 'assistant', 'content': 'some content', 'tool_calls': [{'id': 'call_m6WfL5T37GLbviLKJNvghIOC',...]}

```

would raise

```
UserError: Unhandled item type or structure:
```
