Add sudoku solver with Backtracking Algorithm.

[khalidx3]

Description

• Implemented a Sudoku Solver using the backtracking algorithm in Java.
• Added test cases and ensured proper formatting for consistency.
• Demonstrates the backtracking approach with a classic algorithm problem.

Hacktoberfest

This contribution is intended for Hacktoberfest 2025. I would appreciate it if it could be marked as hacktoberfest-accepted if it meets the repository guidelines.

• 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 code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.68%. Comparing base (3eb521b) to head (5d65233).

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/backtracking/SudokuSolver.java	0.00%	31 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6620      +/-   ##
============================================
- Coverage     75.80%   75.68%   -0.13%     
+ Complexity     5804     5803       -1     
============================================
  Files           705      706       +1     
  Lines         19846    19877      +31     
  Branches       3851     3861      +10     
============================================
- Hits          15045    15044       -1     
- Misses         4218     4249      +31     
- Partials        583      584       +1     

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[DenizAltunkapan]

@khalidx3 please add the corresponding missing test class

[DenizAltunkapan]

@khalidx3 Please fix the failing workflows so I can start the review.

[github-actions]

This pull request has been automatically closed because its workflows or checks failed and it has been inactive for more than 14 days. Please fix the workflows and reopen if you'd like to continue. Merging from main/master alone does not count as activity.