Fixes #5170: Improve msd_type parsing

[tanii1125]

This PR implements enhancement proposed in issue #5170

Summary

EinsteinMSD currently lowercases the msd_type string but does not:

• strip whitespace
• validate the type
• handle mixed-case safely

What This PR Does?

1. Add .strip() before .lower() to handle inputs like " xy ".
2. Add type validation (raise TypeError when msd_type is not a string).
3. Add new tests to cover:
   • whitespace (" xy ")
   • uppercase/mixed case (" Xz ")
   • non-string inputs

All tests pass locally

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📚 Documentation preview 📚: https://mdanalysis--5173.org.readthedocs.build/en/5173/

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.72%. Comparing base (bbcef1b) to head (21bfdf8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5173   +/-   ##
========================================
  Coverage    92.72%   92.72%           
========================================
  Files          180      180           
  Lines        22472    22474    +2     
  Branches      3188     3189    +1     
========================================
+ Hits         20837    20839    +2     
  Misses        1177     1177           
  Partials       458      458           

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