fix: improve mean function with input validation and extended tests

[FatihBARACKILIC]

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Describe your change:

• Fix a bug or typo in an existing algorithm

This PR improves the mean function by:

• Adding input validation:
  • Throws TypeError if input is not an array or contains non-number elements
  • Throws Error if array is empty
• Adding a comprehensive test suite using Vitest, which includes:
  • Empty arrays
  • Invalid elements like null, true, 'string'
  • Floating-point numbers, negative values, large values
  • Mixed integer and float arrays

The function now adheres more strictly to the expected behavior and is more robust against invalid input.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames use the UpperCamelCase (PascalCase) style.
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.88%. Comparing base (1d252d7) to head (2b05c6b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1785   +/-   ##
=======================================
  Coverage   85.87%   85.88%           
=======================================
  Files         377      377           
  Lines       19720    19731   +11     
  Branches     3005     3008    +3     
=======================================
+ Hits        16935    16946   +11     
  Misses       2785     2785           

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