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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (bbcef1b) to head (ced5ece).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5173   +/-   ##
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  Coverage    92.72%   92.72%           
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  Files          180      180           
  Lines        22472    22477    +5     
  Branches      3188     3189    +1     
========================================
+ Hits         20837    20842    +5     
  Misses        1177     1177           
  Partials       458      458           

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[tanii1125]

Added these lines in msd.py , they do -

1. Explicitly checks that msd_type is a string to avoid unexpected AttributeErrors.
2. Uses .strip().lower() to handle inputs with leading/trailing whitespace
   (e.g. " xy ", " Xz "), while preserving existing case-insensitive behavior.

[tanii1125]

Added tests covering msd_type input handling in EinsteinMSD:

• whitespace handling (e.g. " xy ")
• case-insensitive parsing (existing behavior)
• non-string inputs raising TypeError

[Dreamstick9]

Hey! Just checking out the changes.

Since this ecosystem relies heavily on NumPy, users might be passing msd_type as a numpy.str_ (like if they read their config from an array).

This check would strictly raise a TypeError for them, even though it likely worked fine before via duck typing.

It might be safer to either check against (str, np.str_) or just let the .strip().lower() call handle it implicitly so we don't break those workflows.

Also, minor edge case: if the input is just whitespace (e.g. " "), strip() will result in an empty string "". It might be worth raising a ValueError there so it doesn't fail silently downstream?

Nice cleanup on the parsing logic though!

[tanii1125]

@Dreamstick9 thank you for pointing this out !

Since the use of .strip() is in question, I’ll update this PR accordingly:

• keep .lower() while avoiding breaking string-like inputs (e.g. numpy.str_)
• add an explicit check to raise a clear error for empty input

[Dreamstick9]

One more thing I noticed in test_msd.py:

test_msd_type_whitespace_around_valid_value expects a ValueError for " xy ".

Since the PR goal is to support whitespace, shouldn't this test expect success instead of an error?

Currently, this test will likely fail because the code raises a KeyError (due to the unstripped string) rather than the expected ValueError.

[tanii1125]

This is intentional. As discussed in #5170, we decided not to silently normalize whitespace. Inputs like " xy " are treated as invalid, and the test documents that behavior explicitly.