[app-server] feat: expose additional fields on Thread (#6338)

Add the following fields to Thread:

```
    pub preview: String,
    pub model_provider: String,
    pub created_at: i64,
```

Will prob need another PR once this lands:
https://github.com/openai/codex/pull/6337

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -49,7 +49,7 @@ async fn thread_resume_returns_existing_thread() -> Result<()> {
     .await??;
     let ThreadResumeResponse { thread: resumed } =
         to_response::<ThreadResumeResponse>(resume_resp)?;
-    assert_eq!(resumed.id, thread.id);
+    assert_eq!(resumed, thread);
 
     Ok(())
 }