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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.63%. Comparing base (f43ecd5) to head (235d2fa).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6553      +/-   ##
============================================
+ Coverage     75.60%   75.63%   +0.03%     
- Complexity     5724     5733       +9     
============================================
  Files           695      696       +1     
  Lines         19637    19666      +29     
  Branches       3812     3818       +6     
============================================
+ Hits          14846    14874      +28     
  Misses         4213     4213              
- Partials        578      579       +1     

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[Anubhav-pandey004]

hey @TheAlgorithms @DenizAltunkapan can you review it. I would like to contribute.

[DenizAltunkapan]

@Anubhav-pandey004 yes, im currently on vacation, so its hard to review it from my phone. Its going to take some time😆

[DenizAltunkapan]

@Anubhav-pandey004 I think there might be a possible bug when t has duplicate chars (e.g. "AABC"). Please verify by adding a test:

assertEquals("AABBC", MinimumWindowSubstring.minWindow("AAABBC", "AABC"));

[DenizAltunkapan]

Thanks!