Report selected environment connection transitions (#33251)

## What changed

- Track connected and disconnected states across initial remote exec-server connections and reconnection attempts.
- Emit experimental `thread/environment/connected` and `thread/environment/disconnected` app-server notifications for each thread selecting the environment. Each payload identifies the thread and environment; current state is not replayed when a thread starts.
- Stop forwarding connection events when an environment selection is removed or replaced.

## Testing

- Cover connection, disconnection, reconnection, shared-environment notifications, and replacement of a selected environment.

GitOrigin-RevId: 5dd767372363c4a2a8319fc16164be117d5bd20c

codex-rs/core/src/session/session.rs

@@ -1183,6 +1183,7 @@ impl Session {
             for event in events {
                 sess.send_event_raw(event).await;
             }
+            turn_environments.start_connection_event_forwarding(tx_event.clone());
 
             let mcp_startup_cancellation_token = {
                 let mut cancel_guard = sess.services.mcp_startup_cancellation_token.lock().await;

codex-rs/core/src/session/turn.rs

@@ -1552,6 +1552,8 @@ pub(super) fn realtime_text_for_event(msg: &EventMsg) -> Option<(String, Option<
         | EventMsg::AgentReasoningRawContent(_)
         | EventMsg::AgentReasoningSectionBreak(_)
         | EventMsg::SessionConfigured(_)
+        | EventMsg::EnvironmentConnected(_)
+        | EventMsg::EnvironmentDisconnected(_)
         | EventMsg::ThreadGoalUpdated(_)
         | EventMsg::McpStartupUpdate(_)
         | EventMsg::McpStartupComplete(_)