Add sudoku solver with Backtracking Algorithm.

[khalidx3]

Description

• Implemented a Sudoku Solver using the backtracking algorithm in Java.
• Added multiple test cases to verify the solver for different Sudoku puzzles.
• Demonstrates the backtracking approach with a classic algorithm problem.
• File added under Algorithms/Backtracking folder.

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 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.57%. Comparing base (7e29be3) to head (d56855e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/backtracking/SudokuSolver.java	0.00%	38 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6618      +/-   ##
============================================
- Coverage     75.68%   75.57%   -0.11%     
- Complexity     5754     5762       +8     
============================================
  Files           699      701       +2     
  Lines         19725    19789      +64     
  Branches       3826     3841      +15     
============================================
+ Hits          14929    14956      +27     
- Misses         4216     4253      +37     
  Partials        580      580              

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