[FSTORE-1412][Append] Map legacy stddev key to std_dev in stats constructors

After the stddev -> std_dev rename, backend payloads and older callers still
pass the lowercase `stddev` key, which fell into **kwargs and left std_dev as
None — breaking transformation functions that read statistics.<feature>.stddev.
Accept the legacy `stddev` key in both FeatureDescriptiveStatistics and
FeatureTransformationStatistics constructors (std_dev takes precedence).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: javierdlrm

python/hsfs/core/feature_descriptive_statistics.py

@@ -86,7 +86,9 @@ def __init__(
         self._max = max
         self._sum = sum
         self._mean = mean
-        self._std_dev = std_dev
+        # Accept the legacy lowercase ``stddev`` key (e.g. from backend payloads
+        # or older callers) so the rename to ``std_dev`` stays backward compatible.
+        self._std_dev = std_dev if std_dev is not None else kwargs.get("stddev")
         self._percentiles = percentiles
         self._distinctness = distinctness
         self._entropy = entropy

python/hsfs/transformation_statistics.py

@@ -65,7 +65,9 @@ def __init__(
         self._max = max
         self._sum = sum
         self._mean = mean
-        self._std_dev = std_dev
+        # Accept the legacy lowercase ``stddev`` key (e.g. from backend payloads
+        # or older callers) so the rename to ``std_dev`` stays backward compatible.
+        self._std_dev = std_dev if std_dev is not None else kwargs.get("stddev")
         self._percentiles = percentiles
         self._distinctness = distinctness
         self._entropy = entropy