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/schema/json/codex_app_server_protocol.schemas.json

@@ -3973,6 +3973,26 @@
           "title": "McpServer/oauthLogin/completedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "mcpServer/startupStatus/updated"
+              ],
+              "title": "McpServer/startupStatus/updatedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/McpServerStatusUpdatedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "McpServer/startupStatus/updatedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -8570,6 +8590,15 @@
         "title": "McpServerRefreshResponse",
         "type": "object"
       },
+      "McpServerStartupState": {
+        "enum": [
+          "starting",
+          "ready",
+          "failed",
+          "cancelled"
+        ],
+        "type": "string"
+      },
       "McpServerStatus": {
         "properties": {
           "authStatus": {
@@ -8606,6 +8635,29 @@
         ],
         "type": "object"
       },
+      "McpServerStatusUpdatedNotification": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "error": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "name": {
+            "type": "string"
+          },
+          "status": {
+            "$ref": "#/definitions/v2/McpServerStartupState"
+          }
+        },
+        "required": [
+          "name",
+          "status"
+        ],
+        "title": "McpServerStatusUpdatedNotification",
+        "type": "object"
+      },
       "McpToolCallError": {
         "properties": {
           "message": {