Update prepare_mkvtoolnix_path to handle existing paths

[GitBib]

Enhanced prepare_mkvtoolnix_path to check if a string path exists and return it directly if it does. Added corresponding unit tests to ensure proper handling of existing Linux and Windows paths, as well as paths that do not exist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.92%. Comparing base (97b2ea6) to head (4841493).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   65.88%   65.92%   +0.03%     
==========================================
  Files          10       10              
  Lines         894      895       +1     
==========================================
+ Hits          589      590       +1     
  Misses        305      305              

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