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