Use Haiku 4.5 For Subagents instead of Haiku 3.5

Hey,

currently I'm defining one of my subagents like this:

```python
    "subagent": AgentDefinition(
        description="foo",
        tools=tools
        prompt="bar",
        model="haiku",
    ),
```

And from the traces I noticed that this uses `claude-3-5-haiku-20241022`. Would it be possible to allow using haiku 4.5 here instead?

Thanks!