Add FindMiddleOfSinglyLinkedList algorithm and tests

[Hirannya-Mhaisbadwe]

• 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 with clang-format -i --style=file path/to/your/file.java

This PR adds an algorithm to find the middle of a singly linked list, along with unit tests.

Changes include:

• FindMiddleOfSinglyLinkedList.java — implements slow/fast pointer technique to find the middle.
• FindMiddleOfSinglyLinkedListTest.java — JUnit tests covering odd/even lists, single-node, two-node, empty list, and longer lists.

Motivation:

• Finding the middle of a linked list is a common operation for merge sort, palindrome checks, and other list manipulations.
• Adding this improves the completeness of the linked list module.

Fixes #6615

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.70%. Comparing base (7e29be3) to head (e923c86).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6617      +/-   ##
============================================
+ Coverage     75.68%   75.70%   +0.02%     
- Complexity     5754     5755       +1     
============================================
  Files           699      699              
  Lines         19725    19725              
  Branches       3826     3826              
============================================
+ Hits          14929    14933       +4     
+ Misses         4216     4214       -2     
+ Partials        580      578       -2     

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[Hirannya-Mhaisbadwe]

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