Refactor Leaf Node: Immutable Access, Flexible Constructors, and Algorithmic Safety

[Manas-Dikshit]

This PR modifies the Leaf class in the binary tree data structure.

These include:

• Immutable data access: getData() returns a read-only view to avoid making modifications unintentionally while navigating leaves in traversal and comparison.

• Flexible initialization. Constructors to create an empty leaf or from an existing collection have been added.
  Convenience Methods:
  The size() method and isEmpty() method support constant-time queries of their results without showing their internal state.
  Improved equality/hash behavior: This retains the correct equals and hashCode methods, which combine the hashCode from the parent classes with the ordered leaf data.

• Focus on algorithm: Leaf nodes support safe and uniform access for tree traversal, comparisons, and bulk insertions with the intention of preventing side-effects on the underlying array. There is no modification to the tree logic and are all internal to the leaf node for correctness and immutability.