bug: aisdk.MessagesToOpenAI ignores the Content field

If you run:

```
aisdk.MessagesToOpenAI([]aisdk.Message{
{
Role: "user",
Content: "Hello, how are you?",
},
})
```

you'll get out a message with no content.

https://github.com/coder/aisdk-go/blob/5a038b91ecee3d1ad93a0ef8976a50db4f84da20/openai.go#L47

Start in openai.go at the linked line 47 and trace how aisdk.MessagesToOpenAI converts the provided aisdk.Message. Reproduce the example from the issue, then verify that the resulting OpenAI message preserves the Content field instead of being empty.

Written by the indexing model from the issue text.