feat(app-server): add mcpServer/startupStatus/updated notification (#15220)

Exposes the legacy `codex/event/mcp_startup_update` event as an API v2
notification.

The legacy event has this shape:
```
#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
pub struct McpStartupUpdateEvent {
    /// Server name being started.
    pub server: String,
    /// Current startup status.
    pub status: McpStartupStatus,
}

#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
#[serde(rename_all = "snake_case", tag = "state")]
#[ts(rename_all = "snake_case", tag = "state")]
pub enum McpStartupStatus {
    Starting,
    Ready,
    Failed { error: String },
    Cancelled,
}
```

codex-rs/tui_app_server/src/app/app_server_adapter.rs

@@ -492,6 +492,7 @@ fn server_notification_thread_target(
             Some(notification.thread_id.as_str())
         }
         ServerNotification::SkillsChanged(_)
+        | ServerNotification::McpServerStatusUpdated(_)
         | ServerNotification::McpServerOauthLoginCompleted(_)
         | ServerNotification::AccountUpdated(_)
         | ServerNotification::AccountRateLimitsUpdated(_)

codex-rs/tui_app_server/src/chatwidget.rs

@@ -6047,6 +6047,7 @@ impl ChatWidget {
             | ServerNotification::RawResponseItemCompleted(_)
             | ServerNotification::CommandExecOutputDelta(_)
             | ServerNotification::McpToolCallProgress(_)
+            | ServerNotification::McpServerStatusUpdated(_)
             | ServerNotification::McpServerOauthLoginCompleted(_)
             | ServerNotification::AppListUpdated(_)
             | ServerNotification::ContextCompacted(_)