feat: add Rat in a Maze backtracking algorithm

[the-Sunny-Sharma]

Description

Adds a Rat in a Maze implementation using the backtracking technique.

Given an n x n binary maze where 1 = open and 0 = blocked, the
algorithm finds all paths from the top-left (0,0) to the bottom-right
(n-1,n-1) cell. The rat can move in four directions: Down, Left, Right, Up.

Implementation Details

• File: src/main/java/com/thealgorithms/backtracking/RatInAMaze.java
• Approach: Depth-first backtracking with a visited matrix to avoid
  revisiting cells; explores directions in alphabetical order (D, L, R, U)
• Returns: List of all valid path strings (e.g., "DDRDRR")
• Time Complexity: O(4^(n²)) worst case
• Space Complexity: O(n²) for visited matrix and recursion stack

Checklist

• Added implementation in correct package com.thealgorithms.backtracking
• final class with private constructor following repo conventions
• Full Javadoc on class and all public/private methods
• Input validation with IllegalArgumentException for null/empty/non-square maze
• 10 unit tests covering: multiple paths, single path, no path, source
  blocked, destination blocked, single cell open, single cell blocked,
  null maze, empty maze, non-square maze
• All 10 tests pass

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.55%. Comparing base (e814d97) to head (26a7b93).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7418      +/-   ##
============================================
+ Coverage     79.53%   79.55%   +0.01%     
- Complexity     7177     7197      +20     
============================================
  Files           798      799       +1     
  Lines         23474    23501      +27     
  Branches       4617     4624       +7     
============================================
+ Hits          18671    18697      +26     
  Misses         4055     4055              
- Partials        748      749       +1     

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