Add test to cover unit conversion path in DMSReader

[tanii1125]

Summary

Fixes : #3304
This PR adds a small test for DMSReader that exercises the unit-conversion code path when convert_units=True.

Details

• Adds coverage for convert_pos_from_native by triggering position access
• Does not alter reader behavior or logic
• No changes to existing tests

Test verified locally with:

pytest testsuite/MDAnalysisTests/coordinates/test_dms.py::TestDMSReader::test_convert_pos_from_native

Notes

ts.dimensions is intentionally None for DMS files, so this test does not attempt to assert or modify box dimensions.

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📚 Documentation preview 📚: https://mdanalysis--5180.org.readthedocs.build/en/5180/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (9fb8b0a) to head (7882b3b).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5180      +/-   ##
===========================================
- Coverage    92.72%   92.72%   -0.01%     
===========================================
  Files          180      180              
  Lines        22472    22475       +3     
  Branches      3188     3190       +2     
===========================================
+ Hits         20838    20840       +2     
- Misses        1176     1177       +1     
  Partials       458      458              

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[IAlibay]

This test is insufficient, it only flushes the code (and actually doesn't increase coverage for the dimensions path). Checking the size of the array doesn't do much to solve the initial issue.

A proper test here would include actual checks on the numbers to make sure all the results pre and post conversion are as expected. This would include tests for positions, velocities, and dimensions.

[tanii1125]

Thanks for the detailed feedback
I’ll update the test to assert numerical correctness (pre- and post-conversion) for positions, velocities, and dimensions, rather than just triggering the code path.

[RMeli]

@IAlibay concerning velocities I think the DMS files we currently have in MDAnalysisTests have only one frame and no velocities. Do you know of a file with velocities that we have easily available?

[RMeli]

Does the default rtol fails here?

Maybe also worth adding a comment that we expect this to be the same since DMS and MDAnalysis use the same units?

[IAlibay]

Do you know of a file with velocities that we have easily available?

I don't unfortunately, I'm not even sure the current DMS reference file has dimensions, so it's likely the current file isn't testing anything important.