Add Graph Coloring implementation alongside tests

[ethanhaines]

Hello, I saw this repository and wanted to add the classic Graph Coloring backtracking problem, its one I have worked on solving myself before. I added the necessary files in the algorithms directory and inside the test directory, while also updating the README. Let me know if there are any problems with my code, I am a novice contributor on GitHub and am always looking to improve! After my changes, the project successfully built and all tests passed.

• 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

[Copilot]

Pull Request Overview

This PR adds a Graph Coloring Problem solver to the algorithms collection, implementing a backtracking algorithm to assign colors to graph vertices such that no two adjacent vertices share the same color.

• Implements GraphColoringSolver class with backtracking algorithm for the NP-complete graph coloring problem
• Adds comprehensive test suite covering edge cases, various graph types, and validation logic
• Updates README to include the new algorithm in the documentation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
README.md	Adds Graph Coloring entry to the Problems section with link to implementation
Algorithms/Problems/GraphColoring/GraphColoringSolver.cs	New solver implementation with backtracking algorithm, input validation, and comprehensive XML documentation
Algorithms.Tests/Problems/GraphColoring/GraphColoringSolverTests.cs	Comprehensive test suite with 20+ test cases covering edge cases, various graph types (complete, bipartite, cycle, path, star), and validation helpers

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.72%. Comparing base (8bf3d7b) to head (d0d378d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #548      +/-   ##
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+ Coverage   96.70%   96.72%   +0.01%     
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  Files         281      282       +1     
  Lines       11123    11174      +51     
  Branches     1590     1601      +11     
==========================================
+ Hits        10757    10808      +51     
  Misses        232      232              
  Partials      134      134              

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[siriak]

Looks good, thanks!