Complex input for tool failing to parse

**Describe the bug**

I am getting the error:

> Agent error: Error in agent planning: Serde json error: invalid type: map, expected a string

Whenever a tool has an input more complex than a simple `String`.

**Additional context**

This is likely a known issue, and presumably happens because `AgentOutput` looks like this

https://github.com/Abraxas-365/langchain-rust/blob/60c71258d8a40c278f2867f6b4d3871265f6e638/src/agent/chat/output_parser.rs#L14-L18

so when the LLM outputs something like

```json
{
"action": "Some Tool",
"action_input": {
"tool_field1": "Some argument",
"tool_field2": "Other argument",
}
}
```

wouldn't fails to parse since "action_input" is not a `String`