fix(dpmodel): preserve virtual spin atom types

[njzjz-bot]

Closes #5663.

Summary

• preserve negative placeholder types when creating dense and lower-interface spin partners
• use one array-API-safe lookup that maps virtual atom types to zero instead of applying Python negative indexing
• zero placeholder spin displacements, virial corrections, magnetic outputs, and magnetic masks
• add dense, lower, end-to-end invariance, and Array API strict regressions

Why existing tests missed this

Existing spin consistency tests use only nonnegative real atom types, including their lower-interface ghost atoms. Negative-type tests cover base atomic models and neighbor-list builders, where masking happens before type-dependent work, but not SpinModel preprocessing, which runs earlier. Cross-backend spin tests therefore never supplied a nonzero padded spin or asserted that both the real placeholder and its generated spin partner remained negative.

Validation

• pytest source/tests/common/dpmodel/test_spin_model_virtual_types.py source/tests/common/dpmodel/test_finetune_spin.py source/tests/common/dpmodel/test_spin_model_legacy_routing.py -q (12 passed)
• ruff format . (1664 files unchanged on final pass)
• ruff check . (passed)
• git diff --check (passed)

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.31%. Comparing base (6c3b985) to head (f28b33c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5854      +/-   ##
==========================================
- Coverage   78.58%   78.31%   -0.27%     
==========================================
  Files        1050     1050              
  Lines      120637   120641       +4     
  Branches     4356     4354       -2     
==========================================
- Hits        94801    94482     -319     
- Misses      24278    24592     +314     
- Partials     1558     1567       +9     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[njzjz]

Possible reviewers based on changed lines, exact file history, and exact-file review history:

• @wanghan-iapcm — 7 commits on changed files; 42 reviews on exact changed files (deepmd/dpmodel/model/spin_model.py).
• @iProzd — 4 commits on changed files (deepmd/dpmodel/model/spin_model.py).

No review request was made automatically.

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh