Add K-Nearest Neighbors classifier

[poorva0405]

Description

This PR adds a K-Nearest Neighbors (KNN) classifier implementation to the machinelearning package.

Features

• Implements KNN classification using Euclidean distance.
• Supports training through fit().
• Supports prediction for single and batch samples.
• Uses deterministic tie-breaking when multiple classes receive the same number of votes.
• Includes comprehensive input validation.
• Includes JUnit 5 tests covering normal and edge cases.
• Includes Javadocs for all public APIs.

Closes #7562

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.93939% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (7c934ad) to head (3127891).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...ealgorithms/machinelearning/KNearestNeighbors.java	93.93%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7563      +/-   ##
============================================
+ Coverage     80.42%   80.47%   +0.05%     
- Complexity     7457     7488      +31     
============================================
  Files           815      816       +1     
  Lines         24055    24122      +67     
  Branches       4732     4750      +18     
============================================
+ Hits          19346    19412      +66     
  Misses         3945     3945              
- Partials        764      765       +1     

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