[Code scan] Fix CALYPSO nested atom-choice validation to avoid infinite loops #356

Found by Codex global repository scan of deepmodeling/dpgen2 at commit 2679611a3704f5c2646c8cb353e34177518db758.

The nested name_of_atoms validation is intended to reject impossible random choices before entering the selection loop:

The condition compares a set to integer 0:

(set(s) - overlap) == 0

That is always false, so invalid inputs are not rejected. For example, [["A"], ["A"]] can never produce two unique choices. The loop repeatedly chooses duplicate A, breaks the inner loop, and then restarts forever.

Suggested fix: compare against an empty set or use a direct subset check, then add a test that the impossible nested atom-choice case raises ValueError instead of hanging.