Add Dart solution for Binary Tree Inorder Traversal

[rrbharath]

Description

Adds a Dart solution for Binary Tree Inorder Traversal.

The approach uses recursion to visit the left subtree first, then the current node, then the right subtree.

Dependencies: None.

Time Complexity: O(n)
Space Complexity: O(n), due to recursion stack in the worst case
Difficulty: Easy
Tags: Stack, Tree, Depth-First Search, Binary Tree

Put check marks:

Have you made changes in README file ?

• Added problem & solution under correct topic.
• Specified Space & Time complexity.
• Specified difficulty level, tag & Note(if any).

README was not changed in this branch.

How Has This Been Tested?

The solution was reviewed against the expected LeetCode Dart method signature and common sample cases.

• Verified traversal order is left, root, right
• Verified null root returns an empty list

Make sure all below guidelines are followed else PR will get Reject:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code so that it is easy to understand
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules