Add algorithm to find middle of singly linked list with tests

[koushik-sai]

This PR adds an enhancement to the linked list data structures by:

• Implementing an algorithm to find the middle node of a singly linked list using the slow and fast pointer technique
• Documenting behavior for even-length lists (returns second middle)
• Adding unit tests covering empty, single-node, odd-length, and even-length lists

Fixes #6617

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.92%. Comparing base (d9f2ac8) to head (0f197fa).

Files with missing lines	Patch %	Lines
...structures/lists/FindMiddleOfSinglyLinkedList.java	92.30%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7166      +/-   ##
============================================
+ Coverage     78.90%   78.92%   +0.01%     
- Complexity     6853     6858       +5     
============================================
  Files           769      770       +1     
  Lines         22668    22681      +13     
  Branches       4456     4457       +1     
============================================
+ Hits          17887    17900      +13     
- Misses         4067     4068       +1     
+ Partials        714      713       -1     

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[koushik-sai]

Thanks for reviewing this! 🙌
After further inspection, I noticed that the middle-finding logic already exists in SinglyLinkedList and is already covered by unit tests. Since this enhancement is already addressed, I’m closing the PR to avoid duplication.