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 (0a67724).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5180      +/-   ##
===========================================
- Coverage    92.72%   92.72%   -0.01%     
===========================================
  Files          180      180              
  Lines        22472    22472              
  Branches      3188     3188              
===========================================
- Hits         20838    20837       -1     
- 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.