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,
}
```
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