Add sudoku solver

[Raghu0703]

Description

This PR adds a Sudoku Solver implementation using the Backtracking algorithm as requested in issue #6929.

Changes

• Added SudokuSolver.java in src/main/java/com/thealgorithms/backtracking/

  • Implements backtracking algorithm to solve 9×9 Sudoku puzzles
  • Includes isValid() method to verify Sudoku constraints (row, column, 3×3 subgrid)
  • Includes solveSudoku() method with recursive backtracking logic
  • Includes printBoard() helper method for formatted output
  • Provides example usage in main() method

• Added SudokuSolverTest.java in src/test/java/com/thealgorithms/backtracking/

  • Tests solvable Sudoku puzzle with expected solution verification
  • Tests unsolvable Sudoku puzzle (invalid constraints)
  • Tests already solved Sudoku
  • Tests empty Sudoku board
  • Tests difficult Sudoku puzzle with fewer clues

Algorithm Details

• Approach: Depth-first search with backtracking
• Time Complexity: O(9^m) where m is the number of empty cells
• Space Complexity: O(m) for the recursion stack
• Reference: https://en.wikipedia.org/wiki/Sudoku_solving_algorithms#Backtracking

Testing

✅ All new tests pass: mvn test -Dtest=SudokuSolverTest
✅ Verified with multiple Sudoku puzzles
✅ Code follows repository formatting standards
✅ Includes proper documentation and comments

Checklist

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL