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/src/codex_message_processor.rs

@@ -2455,6 +2455,7 @@ impl CodexMessageProcessor {
                 let turn = Turn {
                     id: turn_id.clone(),
                     items: vec![],
+                    error: None,
                     status: TurnStatus::InProgress,
                 };
 
@@ -2496,6 +2497,7 @@ impl CodexMessageProcessor {
         Turn {
             id: turn_id,
             items,
+            error: None,
             status: TurnStatus::InProgress,
         }
     }