Add app-server MCP event streaming (#39761)

## What changed

- Add experimental `mcpServer/event/stream/start` and `mcpServer/event/stream/stop` requests for hosted apps, plus `mcpServer/event/stream/notification` forwarding.
- Scope subscriptions to the owning app-server connection and subscribed thread, enforce unique IDs and a per-connection limit, and clean them up when the thread is unsubscribed or the connection closes.
- Wait for the MCP active notification before completing startup, retry streams that close immediately, and terminate them when authentication or hosted runtime ownership changes.

## Testing

- Add an app-server integration test covering activation, event forwarding, duplicate subscription rejection, and explicit cancellation.

GitOrigin-RevId: 8a6fc1615adfc5af7e67def3b824fa5909ab3e7b

codex-rs/tui/src/chatwidget/protocol.rs

@@ -222,6 +222,7 @@ impl ChatWidget {
             | ServerNotification::CommandExecOutputDelta(_)
             | ServerNotification::ProcessOutputDelta(_)
             | ServerNotification::ProcessExited(_)
+            | ServerNotification::McpServerEventStream(_)
             | ServerNotification::FileChangePatchUpdated(_)
             | ServerNotification::McpToolCallProgress(_)
             | ServerNotification::McpServerOauthLoginCompleted(_)