chore: improve serialization of ServerNotification (#3193)

This PR introduces introduces a new
`OutgoingMessage::AppServerNotification` variant that is designed to
wrap a `ServerNotification`, which makes the serialization more
straightforward compared to
`OutgoingMessage::Notification(OutgoingNotification)`. We still use the
latter for serializing an `Event` as a `JSONRPCMessage::Notification`,
but I will try to get away from that in the near future.

With this change, now the generated TypeScript type for
`ServerNotification` is:

```typescript
export type ServerNotification =
  | { "method": "authStatusChange", "params": AuthStatusChangeNotification }
  | { "method": "loginChatGptComplete", "params": LoginChatGptCompleteNotification };
```

whereas before it was:

```typescript
export type ServerNotification =
  | { type: "auth_status_change"; data: AuthStatusChangeNotification }
  | { type: "login_chat_gpt_complete"; data: LoginChatGptCompleteNotification };
```

Once the `Event`s are migrated to the `ServerNotification` enum in Rust,
it should be considerably easier to work with notifications on the
TypeScript side, as it will be possible to `switch (message.method)` and
check for exhaustiveness.

Though we will probably need to introduce:

```typescript
export type ServerMessage = ServerRequest | ServerNotification;
```

and then we still need to group all of the `ServerResponse` types
together, as well.

codex-rs/mcp-server/src/codex_message_processor.rs

@@ -815,9 +815,9 @@ impl CodexMessageProcessor {
                         };
 
                         // For now, we send a notification for every event,
-                        // JSON-serializing the `Event` as-is, but we will move
-                        // to creating a special enum for notifications with a
-                        // stable wire format.
+                        // JSON-serializing the `Event` as-is, but these should
+                        // be migrated to be variants of `ServerNotification`
+                        // instead.
                         let method = format!("codex/event/{}", event.msg);
                         let mut params = match serde_json::to_value(event.clone()) {
                             Ok(serde_json::Value::Object(map)) => map,