Added Sudoku Solver with backtracking algorithm.

[khalidx3]

What does this PR do?

• Adds a new backtracking algorithm: SudokuSolver.java.

• Implements a recursive backtracking solution to solve standard 9×9 Sudoku puzzles.

• Includes a helper function printBoard() for board visualization.

• 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

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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.55%. Comparing base (7e29be3) to head (f554ae0).

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/backtracking/SudokuSolver.java	0.00%	39 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6607      +/-   ##
============================================
- Coverage     75.68%   75.55%   -0.14%     
  Complexity     5754     5754              
============================================
  Files           699      700       +1     
  Lines         19725    19764      +39     
  Branches       3826     3839      +13     
============================================
+ Hits          14929    14932       +3     
- Misses         4216     4253      +37     
+ Partials        580      579       -1     

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