`chat.session.prompt` should use the last used model like the opencode cli. Instead it defaults to `gpt-5-nano`

`chat.session.prompt` should use the last used model like the opencode cli. Instead it defaults to `gpt-5-nano`

When doing

```tsx
await client.session
  .prompt({
    path: { id: session.data.id },
    body: {
      parts: [{ type: 'text', text: message }],˙
    },
  })
```