Update default handler in Langfuse Tracer to serialize ToolCallResult properly

[sjrl]

In the example shown below we can see that tool_messages isn't a JSON dict but a string due to it not being JSON serializable.

This is most likely due to State not having a to_dict method. A to_dict method should be added to State so the tool_messages can be represented as a dict in Langfuse.

[julian-risch]

Closing because we track the issue now in deepset-ai/haystack#9286