[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_list.rs

@@ -102,6 +102,11 @@ async fn thread_list_pagination_next_cursor_none_on_last_page() -> Result<()> {
         next_cursor: cursor1,
     } = to_response::<ThreadListResponse>(page1_resp)?;
     assert_eq!(data1.len(), 2);
+    for thread in &data1 {
+        assert_eq!(thread.preview, "Hello");
+        assert_eq!(thread.model_provider, "mock_provider");
+        assert!(thread.created_at > 0);
+    }
     let cursor1 = cursor1.expect("expected nextCursor on first page");
 
     // Page 2: with cursor → expect next_cursor None when no more results.
@@ -122,6 +127,11 @@ async fn thread_list_pagination_next_cursor_none_on_last_page() -> Result<()> {
         next_cursor: cursor2,
     } = to_response::<ThreadListResponse>(page2_resp)?;
     assert!(data2.len() <= 2);
+    for thread in &data2 {
+        assert_eq!(thread.preview, "Hello");
+        assert_eq!(thread.model_provider, "mock_provider");
+        assert!(thread.created_at > 0);
+    }
     assert_eq!(cursor2, None, "expected nextCursor to be null on last page");
 
     Ok(())
@@ -200,6 +210,11 @@ async fn thread_list_respects_provider_filter() -> Result<()> {
     let ThreadListResponse { data, next_cursor } = to_response::<ThreadListResponse>(resp)?;
     assert_eq!(data.len(), 1);
     assert_eq!(next_cursor, None);
+    let thread = &data[0];
+    assert_eq!(thread.preview, "X");
+    assert_eq!(thread.model_provider, "other_provider");
+    let expected_ts = chrono::DateTime::parse_from_rfc3339("2025-01-02T11:00:00Z")?.timestamp();
+    assert_eq!(thread.created_at, expected_ts);
 
     Ok(())
 }