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/thread_history.rs

@@ -1,5 +1,6 @@
 use crate::protocol::v2::ThreadItem;
 use crate::protocol::v2::Turn;
+use crate::protocol::v2::TurnError;
 use crate::protocol::v2::TurnStatus;
 use crate::protocol::v2::UserInput;
 use codex_protocol::protocol::AgentReasoningEvent;
@@ -142,6 +143,7 @@ impl ThreadHistoryBuilder {
         PendingTurn {
             id: self.next_turn_id(),
             items: Vec::new(),
+            error: None,
             status: TurnStatus::Completed,
         }
     }
@@ -190,6 +192,7 @@ impl ThreadHistoryBuilder {
 struct PendingTurn {
     id: String,
     items: Vec<ThreadItem>,
+    error: Option<TurnError>,
     status: TurnStatus,
 }
 
@@ -198,6 +201,7 @@ impl From<PendingTurn> for Turn {
         Self {
             id: value.id,
             items: value.items,
+            error: value.error,
             status: value.status,
         }
     }