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/mcp-server/tests/suite/codex_message_processor_flow.rs

@@ -90,6 +90,7 @@ async fn test_codex_jsonrpc_conversation_flow() {
     let NewConversationResponse {
         conversation_id,
         model,
+        reasoning_effort: _,
         rollout_path: _,
     } = new_conv_resp;
     assert_eq!(model, "mock-model");