fix: workaround freqtrade 2026.4 date column dtype regression

fix: workaround freqtrade 2026.4 date column dtype regression

Freqtrade 2026.4 (commit 2c5dc72) changed feather/parquet handlers to
use .dt.as_unit("ms") instead of to_datetime(col, unit="ms", utc=True).
This breaks when data files store dates as int64 epoch-ms, causing
AttributeError in feature_engineering_standard.

Use pd.to_datetime(col, utc=True) defensively to handle both int64 and
datetime inputs.

Ref: https://github.com/freqtrade/freqtrade/issues/13107