In addition to strings (which we already supported), OpenResponses
supports arrays of text content, image content, or file content (see
<https://www.openresponses.org/reference#object-FunctionCallOutput-title>).
We were missing support for these arrays, which caused unmarshal errors
like
```
json: cannot unmarshal array into Go struct field ResponsesFunctionCallOutput.output of type string
```
This change adds support for text content and image content, as those
are more straightforwardly mappable to Ollama message formats (though
image and text interleaving is lost), but it's less clear what to do for
files. In the future we can partially support this by inlining
reasonably sized text files, but wanted to get this change out first.
Fixes: #15250
