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/mcp-server/src/codex_tool_runner.rs

@@ -333,6 +333,8 @@ async fn run_codex_tool_session_inner(
                     EventMsg::AgentReasoningRawContent(_)
                     | EventMsg::TurnStarted(_)
                     | EventMsg::ThreadSettingsApplied(_)
+                    | EventMsg::EnvironmentConnected(_)
+                    | EventMsg::EnvironmentDisconnected(_)
                     | EventMsg::TokenCount(_)
                     | EventMsg::AgentReasoning(_)
                     | EventMsg::AgentReasoningSectionBreak(_)