fix(dataflows): add missing pandas import in y_finance.py

The _get_stock_stats_bulk() function uses pd.isna() but pandas was not
imported in y_finance.py. It worked by accident because stockstats_utils
was imported first and loaded pandas into the module namespace. This
fragile implicit dependency breaks if import order changes.

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