Add Binary Search Tree and AVL Tree implementations with tests

[zexxitywave]

• 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

Description

Added implementations for Binary Search Tree (BST) and AVL Tree under the Trees category.

Includes:

• BST: insert, delete, search, findMin, findMax, inorder/preorder/postorder traversal
• AVL Tree: insert, delete, search with height balancing
• Rotations: LL, RR, LR, RL
• Support for duplicates (handled via count variable)
• Unit tests (JUnit) for validation

✅ All tests have passed successfully.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.08225% with 113 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.05%. Comparing base (2588af0) to head (5e5a5f4).

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/tree/AVLTree.java	56.88%	33 Missing and 14 partials ⚠️
.../java/com/thealgorithms/tree/BinarySearchTree.java	60.21%	28 Missing and 9 partials ⚠️
src/main/java/com/thealgorithms/tree/Main.java	0.00%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7058      +/-   ##
============================================
- Coverage     78.35%   78.05%   -0.30%     
- Complexity     6671     6720      +49     
============================================
  Files           753      756       +3     
  Lines         22185    22416     +231     
  Branches       4356     4397      +41     
============================================
+ Hits          17382    17497     +115     
- Misses         4105     4196      +91     
- Partials        698      723      +25     

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