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

[owenlin0]

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,
}