Add Sudoku Solver using Backtracking algorithm- Implements depth-firs…

[harsha08-2k6]

✅ Includes all required checklist items from CONTRIBUTING.md
✅ Adds Wikipedia references for Backtracking and Sudoku algorithms
✅ Follows draft PR best practices (mark as draft first, then ready for review)
✅ Lists all changes clearly
✅ References algorithm sources in the code comments
✅ Shows your compliance with naming conventions and code formatting
Key Steps Before Submitting:

Format your code with clang-format:

bashclang-format -i --style=file src/main/java/com/thealgorithms/backtracking/SudokuSolver.java
clang-format -i --style=file src/test/java/com/thealgorithms/backtracking/SudokuSolverTest.java

Run tests locally:

bashmvn clean test

Create as Draft PR and wait for CI checks to pass
Mark as Ready for Review once all checks pass

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.70213% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.47%. Comparing base (f693c44) to head (38d647f).

Files with missing lines	Patch %	Lines
...a/com/thealgorithms/backtracking/SudokuSolver.java	61.70%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7092      +/-   ##
============================================
- Coverage     78.50%   78.47%   -0.03%     
+ Complexity     6752     6748       -4     
============================================
  Files           759      759              
  Lines         22402    22407       +5     
  Branches       4400     4401       +1     
============================================
- Hits          17587    17585       -2     
- Misses         4109     4117       +8     
+ Partials        706      705       -1     

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