Add Minimum Window Substring algorithm in Sliding Window package

[Anubhav-pandey004]

Description

This PR adds an implementation of the Minimum Window Substring problem to the com.thealgorithms.slidingwindow package.
The algorithm finds the smallest substring in s that contains all characters of t.

Time Complexity: O(n) (where n = length of s)

Space Complexity: O(|charset|) (for HashMaps storing frequencies)

Code Summary

Implemented Solution.minWindow(String s, String t) method.

Uses two hash maps:

tFreq → stores required character frequencies of t.

windowFreq → tracks character frequencies in current window.

Expands right pointer until all required characters are included.

Shrinks from left to minimize window length while still valid.

Returns the smallest valid substring, or "" if none exists.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.06%. Comparing base (ef681e8) to head (6ab6e25).

Files with missing lines	Patch %	Lines
...gorithms/slidingwindow/MinimumWindowSubstring.java	0.00%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6550      +/-   ##
============================================
- Coverage     75.18%   75.06%   -0.12%     
+ Complexity     5615     5614       -1     
============================================
  Files           690      691       +1     
  Lines         19408    19437      +29     
  Branches       3756     3762       +6     
============================================
- Hits          14592    14591       -1     
- Misses         4248     4277      +29     
- Partials        568      569       +1     

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[DenizAltunkapan]

Please always add a test class to your corresponding implementatkon @Anubhav-pandey004