[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-protocol/src/protocol/v2.rs

@@ -354,6 +354,11 @@ pub struct ThreadCompactResponse {}
 #[ts(export_to = "v2/")]
 pub struct Thread {
     pub id: String,
+    /// Usually the first user message in the thread, if available.
+    pub preview: String,
+    pub model_provider: String,
+    /// Unix timestamp (in seconds) when the thread was created.
+    pub created_at: i64,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]