fix: replace deprecated pandas fillna(method=) with ffill()/bfill() (#1987)

* fix: replace deprecated pandas fillna(method=) with ffill()/bfill()

  Replace deprecated fillna(method="ffill"/"bfill") calls with modern
  pandas ffill() and bfill() methods to fix FutureWarnings in pandas 2.x.

  Also includes black formatting fixes for compliance.

  This addresses the pandas deprecation warnings portion of issue #1981.
  Other issues (date parsing, type conversion, timezone handling) will be
  addressed in separate commits.

  Fixes:
  - Yahoo collector: 2 instances in calc_change() and adjusted_price()
  - BaoStock collector: 1 instance in calc_change()
  - Core utils: resam.py fillna operations
  - Backtest: profit_attribution.py stock data processing
  - High-freq ops: FFillNan and BFillNan operators
  - Position analysis: parse_position.py weight processing

  Partially addresses GitHub issue #1981

* lint with black

* lint with black

* limit minimum version of pandas

* limit minimum version of pandas

---------

Co-authored-by: Linlang <Lv.Linlang@hotmail.com>

qlib/utils/mod.py

@@ -161,7 +161,6 @@ def init_instance_by_config(
             # path like 'file:///<path to pickle file>/obj.pkl'
             pr = urlparse(config)
             if pr.scheme == "file":
-
                 # To enable relative path like file://data/a/b/c.pkl.  pr.netloc will be data
                 path = pr.path
                 if pr.netloc != "":