[Bug]: llm function calls in chat-message[] don't export to dataset

### What happened?

I have an array of chat messages that include function calls that I append to a dataset. Data studio shows an empty cell for function calls.

### What was the expected functionality?

Expected to see the function call JSON w/ name of function and arguments.

### Describe your environment

Windows 11, Rivet 1.7.7

### Relevant log output

_No response_

### Relevant screenshots

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Start by locating the code that exports chat-message[] values into datasets and handling for function calls. Reproduce the issue with a message containing a function name and arguments, then inspect the resulting dataset in Data Studio. Done means the cell shows the function-call JSON instead of being empty.

Written by the indexing model from the issue text.