feat: include reasoning_effort in NewConversationResponse (#3506)

`ClientRequest::NewConversation` picks up the reasoning level from the user's defaults in `config.toml`, so it should be reported in `NewConversationResponse`.

codex-rs/core/src/codex.rs

@@ -505,6 +505,7 @@ impl Session {
             msg: EventMsg::SessionConfigured(SessionConfiguredEvent {
                 session_id: conversation_id,
                 model,
+                reasoning_effort: model_reasoning_effort,
                 history_log_id,
                 history_entry_count,
                 initial_messages,