fix: add missing pandas import and fix JSON Unicode encoding

- Fix 'name pd is not defined' error in _get_stock_stats_bulk by adding missing pandas import
- Add ensure_ascii=False to json.dump() to properly display Chinese characters in log files

tradingagents/dataflows/y_finance.py

@@ -1,6 +1,7 @@
 from typing import Annotated
 from datetime import datetime
 from dateutil.relativedelta import relativedelta
+import pandas as pd
 import yfinance as yf
 import os
 from .stockstats_utils import StockstatsUtils, _clean_dataframe, yf_retry, load_ohlcv, filter_financials_by_date

tradingagents/graph/trading_graph.py

@@ -267,7 +267,7 @@ class TradingAgentsGraph:
             "w",
             encoding="utf-8",
         ) as f:
-            json.dump(self.log_states_dict, f, indent=4)
+            json.dump(self.log_states_dict, f, indent=4, ensure_ascii=False)
 
     def reflect_and_remember(self, returns_losses):
         """Reflect on decisions and update memory based on returns."""