fix: catch AttributeError in validate_data to allow Union fallthrough for non-DataFrame inputs (#993)

* fix: catch AttributeError in validate_data for non-DataFrame Union types

PUT /v1/prediction/import/{provider_id} crashed with AttributeError
when Union validator tried PydanticDateTimeDataFrame validation on
scalar float/int values. Instead of falling through to DateTimeData
or dict, the exception propagated as 500. Fixes #992.

* fix: correct indentation of ValueError in validate_data

---------

Co-authored-by: Normann <github@koldrack.com>

src/akkudoktoreos/core/pydantic.py

@@ -1023,7 +1023,12 @@ class PydanticDateTimeDataFrame(PydanticBaseModel):
             return v
 
         # Validate consistent columns
-        columns = set(next(iter(v.values())).keys())
+        try:
+            columns = set(next(iter(v.values())).keys())
+        except AttributeError:
+            raise ValueError(
+                "Data values must be dicts (DataFrame format), not scalars. Use DateTimeData or DateTimeSeries format instead."
+            )
         if not all(set(row.keys()) == columns for row in v.values()):
             raise ValueError("All rows must have the same columns")