fix: Handle numpy.ndarray from Arrow/Athena in Array(String) materialisation

[Abhishek8108]

Summary

Closes #6325

Materialising feature views with Array(String) columns via the Athena offline store fails with TypeError or ValueError. Arrow/Athena deserialises these columns as numpy.ndarray with object dtype instead of plain Python lists, and two code paths in type_map.py do not handle this.

Root causes

1. _validate_collection_item_types — TypeError on None elements inside ndarrays

Nullable Arrow columns embed None elements inside the ndarray. The validator iterates over the ndarray directly and checks type(item) in valid_types. For STRING_LIST, valid_types = [np.str_, str], and type(None) is not in that set, so it raises TypeError: bad argument type for built-in operation.

2. _convert_list_values_to_proto generic path — TypeError from protobuf

The generic list conversion calls StringList(val=value) where value is a numpy.ndarray. Protobuf rejects non-list inputs with TypeError: bad argument type for built-in operation.

Fix

• In _validate_collection_item_types: coerce ndarray to a plain Python list via .tolist() before element-level checks, and skip None elements (nullable elements are valid in Arrow columns).
• In the generic conversion path of _convert_list_values_to_proto: apply the same .tolist() coercion so protobuf always receives a plain Python list. None elements are stripped (protobuf fixed-type lists cannot hold nulls).

The fix covers all three failure modes from #6325:

• np.array(['a', 'b', 'c']) → ['a', 'b', 'c'] ✓
• np.array(['a', None, 'c']) → ['a', 'c'] (None stripped) ✓
• np.array([]) → [] ✓

Changes

• sdk/python/feast/type_map.py — two targeted edits, no API surface change
• sdk/python/tests/unit/test_type_map.py — four new unit tests in TestAthenaArrayStringConversion

Test plan

• TestAthenaArrayStringConversion::test_string_list_from_ndarray — plain ndarray converts correctly
• TestAthenaArrayStringConversion::test_string_list_from_ndarray_with_none_elements — None elements stripped, no TypeError
• TestAthenaArrayStringConversion::test_string_list_from_empty_ndarray — empty ndarray yields empty list, no ValueError
• TestAthenaArrayStringConversion::test_string_list_mixed_null_and_ndarray_rows — mix of null rows and ndarray rows
• ruff format --check and ruff check — clean on both modified files
• Existing test_type_map.py tests unaffected