fix: remove serde(flatten) annotation for TurnError (#7499)

The problem with using `serde(flatten)` on Turn status is that it
conditionally serializes the `error` field, which is not the pattern we
want in API v2 where all fields on an object should always be returned.

```
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
#[serde(rename_all = "camelCase")]
#[ts(export_to = "v2/")]
pub struct Turn {
    pub id: String,
    /// Only populated on a `thread/resume` response.
    /// For all other responses and notifications returning a Turn,
    /// the items field will be an empty list.
    pub items: Vec<ThreadItem>,
    #[serde(flatten)]
    pub status: TurnStatus,
}

#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
#[serde(tag = "status", rename_all = "camelCase")]
#[ts(tag = "status", export_to = "v2/")]
pub enum TurnStatus {
    Completed,
    Interrupted,
    Failed { error: TurnError },
    InProgress,
}
```

serializes to:
```
{
  "id": "turn-123",
  "items": [],
  "status": "completed"
}

{
  "id": "turn-123",
  "items": [],
  "status": "failed",
  "error": {
    "message": "Tool timeout",
    "codexErrorInfo": null
  }
}
```

Instead we want:
```
{
  "id": "turn-123",
  "items": [],
  "status": "completed",
  "error": null
}

{
  "id": "turn-123",
  "items": [],
  "status": "failed",
  "error": {
    "message": "Tool timeout",
    "codexErrorInfo": null
  }
}
```

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -877,8 +877,9 @@ pub struct Turn {
     /// For all other responses and notifications returning a Turn,
     /// the items field will be an empty list.
     pub items: Vec<ThreadItem>,
-    #[serde(flatten)]
     pub status: TurnStatus,
+    /// Only populated when the Turn's status is failed.
+    pub error: Option<TurnError>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS, Error)]
@@ -900,12 +901,12 @@ pub struct ErrorNotification {
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
-#[serde(tag = "status", rename_all = "camelCase")]
-#[ts(tag = "status", export_to = "v2/")]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
 pub enum TurnStatus {
     Completed,
     Interrupted,
-    Failed { error: TurnError },
+    Failed,
     InProgress,
 }