Enhanced FastSearcherTests with additional test cases

[gmottajr]

Enhanced FastSearcherTests with additional test cases and improved assertions using FluentAssertions.

Introduced new test cases to cover additional scenarios:

•     FindIndex_ItemPresentInArrayOfDuplicates_IndexCorrect: Verifies correct behavior when searching for an item in an array with all duplicate values.
  

•     FindIndex_ItemMissingInArrayOfDuplicates_ItemNotFoundExceptionThrown: Ensures an exception is thrown when searching for an absent item in an array of duplicates.
  

•     FindIndex_ItemOutOfRange_ItemNotFoundExceptionThrown: Confirms exceptions are raised for items smaller or larger than the array's range.
  

  Replaced NUnit's traditional Assert syntax with FluentAssertions for:

    Improved readability.
    Consistent assertion style.
    Expressive exception validation using act.Should().Throw<Exception>().
  

Maintained edge case validation for empty arrays, out-of-range values, and general functionality.

These updates refine test clarity, improve maintainability, and ensure comprehensive coverage of the FastSearcher class.

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (046f26e) to head (f94da60).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #501   +/-   ##
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  Coverage   95.16%   95.16%           
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  Files         267      267           
  Lines       10848    10848           
  Branches     1527     1527           
=======================================
  Hits        10323    10323           
  Misses        400      400           
  Partials      125      125           

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[siriak]

Looks good, thanks!