Refractor Binary Search Tree to mordern C++ using STL and OOP

[shreyash24-tech]

Description of Change

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
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Notes:

[shreyash24-tech]

This PR refactors the existing Binary Search Tree implementation to modern C++ style as mentioned in the TODO comment in the original file. The previous version used a C-style struct design with global variables and a manual queue implementation.

In this update, I converted the implementation into a proper class-based structure, removed the global queue, and replaced it with std::queue from STL. I also corrected the deletion logic to properly handle root deletion and all BST delete cases. Additionally, I replaced NULL with nullptr, improved memory management by adding a destructor, and enhanced overall code readability and safety.

The functionality of the BST remains the same, but the structure now follows modern C++ practices.

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