fix: update tail when inserting at the end of LinkedList

[yorunohoshi]

Description

In the LinkedList.insert() method, the tail pointer was not being updated when a new node was inserted at the very end of the list (at index = length). This caused an inconsistency where tail remained pointed to the second-to-last node.

Impact

If append() is called immediately after such an insert(), the newly appended node would overwrite the node previously inserted by insert(), leading to data loss.

Changes

• Updated insert() to check if the new node is inserted at the tail position and update this.tail accordingly.
• Added a logic to handle tail updates even when inserting into an empty list or out of bounds.
• Added a new test case in LinkedList.test.js to verify tail consistency and prevent regression.

Checklist

• My code follows the code style of this project (npm run lint passed).
• I have performed a self-review of my own code.
• I have added a test that proves my fix is effective.
• All new and existing tests passed (npm test passed).