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/app-server-protocol/src/protocol/common.rs

@@ -905,6 +905,7 @@ server_notification_definitions! {
     ServerRequestResolved => "serverRequest/resolved" (v2::ServerRequestResolvedNotification),
     McpToolCallProgress => "item/mcpToolCall/progress" (v2::McpToolCallProgressNotification),
     McpServerOauthLoginCompleted => "mcpServer/oauthLogin/completed" (v2::McpServerOauthLoginCompletedNotification),
+    McpServerStatusUpdated => "mcpServer/startupStatus/updated" (v2::McpServerStatusUpdatedNotification),
     AccountUpdated => "account/updated" (v2::AccountUpdatedNotification),
     AccountRateLimitsUpdated => "account/rateLimits/updated" (v2::AccountRateLimitsUpdatedNotification),
     AppListUpdated => "app/list/updated" (v2::AppListUpdatedNotification),

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -5002,6 +5002,25 @@ pub struct McpServerOauthLoginCompletedNotification {
     pub error: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum McpServerStartupState {
+    Starting,
+    Ready,
+    Failed,
+    Cancelled,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct McpServerStatusUpdatedNotification {
+    pub name: String,
+    pub status: McpServerStartupState,
+    pub error: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]