fix: use identity check for type comparison and avoid mutable default argument - #6742

Summary

Two small lint/correctness fixes:

1. sdk/python/feast/diff/registry_diff.py — assert type(current_proto) == type(new_proto) compares type objects with ==. Newer ruff flags this as E721 (which is inside the E7 family this repo selects in pyproject.toml), and identity is the intended semantics for type objects:

# Before
assert type(current_proto) == type(new_proto)
# After
assert type(current_proto) is type(new_proto)

2. sdk/python/feast/api/registry/rest/rest_utils.py — validate_or_set_default_sorting_params(sort_by_options: List[str] = []) uses a mutable default argument (ruff B006). Changed to the Optional[...] = None sentinel. Behavior is unchanged: the inner dependency only ever checks truthiness (if not sort_by_options:), which treats None and [] identically.

Testing

No behavior change — python -m py_compile and ruff check --select E721,B006 pass on both files. The registry_diff.py change preserves the file's existing line endings.

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