[FEAT] Implement Flattening MultiLevel LinkedList

[Priyanshu1303d]

This pull request adds an implementation for the Flattening a Multilevel Linked List.

The problem involves a linked list where each node has a next pointer and a child pointer. The child pointer points to a separate, sorted linked list. This algorithm recursively merges these lists into a single, fully sorted list connected by the child pointers.

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• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted correctly.

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This submission includes:
FlattenMultilevelLinkedList.java: The core implementation.
FlattenMultilevelLinkedListTest.java: A corresponding set of unit tests to ensure correctness.

All local tests pass successfully.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.78%. Comparing base (9484c7e) to head (b3a93bd).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6670      +/-   ##
============================================
+ Coverage     75.75%   75.78%   +0.03%     
- Complexity     5772     5780       +8     
============================================
  Files           703      704       +1     
  Lines         19777    19798      +21     
  Branches       3832     3834       +2     
============================================
+ Hits          14982    15004      +22     
  Misses         4215     4215              
+ Partials        580      579       -1     

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