fix: reject impossible CALYPSO atom choices

[njzjz-bot]

Summary

• compare nested atom-choice differences against an empty set correctly
• raise ValueError before entering the random selection loop
• add a regression test for [["Li"], ["Li"]]

Tests

• PYTHONPATH=tests python -m unittest -v tests.exploration.test_make_task_group_from_config.TestMakeCalyTaskGroupFromConfig.test_rejects_impossible_random_atom_choices tests.exploration.test_make_task_group_from_config.TestMakeCalyTaskGroupFromConfig.test_caly_task_group
• ruff format --check dpgen2/exploration/task/caly_task_group.py tests/exploration/test_make_task_group_from_config.py
• isort --check-only dpgen2/exploration/task/caly_task_group.py tests/exploration/test_make_task_group_from_config.py
• git diff --check

Closes #356

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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Run ID: 312172fa-88f6-495e-b98f-c1dadf1af8c7

📥 Commits

Reviewing files that changed from the base of the PR and between 6b01f29 and d3ca156.

📒 Files selected for processing (2)

• dpgen2/exploration/task/caly_task_group.py
• tests/exploration/test_make_task_group_from_config.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.45%. Comparing base (6b01f29) to head (d3ca156).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
+ Coverage   84.43%   84.45%   +0.01%     
==========================================
  Files         104      104              
  Lines        6110     6110              
==========================================
+ Hits         5159     5160       +1     
+ Misses        951      950       -1     

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Retracted. This review was produced without running the mandated /code-review fan-out (the loop skill's section 2); the substitute process used instead has since been shown to miss findings and, in one case, to state a verified-sounding falsehood. Re-reviewing properly.