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/exec/src/event_processor_with_human_output.rs

@@ -520,6 +520,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 let SessionConfiguredEvent {
                     session_id: conversation_id,
                     model,
+                    reasoning_effort: _,
                     history_log_id: _,
                     history_entry_count: _,
                     initial_messages: _,