[26/N] Add unit tests for LinkedList, DoublyLinkedList, and CircularlyLinkedList algorithms

[karkir0003]

🥞 Stack

1. ⬅️ [1/N] Update CI to run frontend unit tests #91 (Update CI to run frontend unit tests)
2. ⬅️ [2/N] Configure React Testing Library and Setup Tests #92 (Configure React Testing Library and Setup Tests)
3. ⬅️ [3/N] Add unit tests for SideButton component #94 (Add unit tests for SideButton component)
4. ⬅️ [4/N] Add unit tests for SearchFilter component #95 (Add unit tests for SearchFilter component)
5. ⬅️ [5/N] Add unit tests for Header and Footer components #96 (Add unit tests for Header and Footer components)
6. ⬅️ [6/N] Add unit tests for Blob mascot component #97 (Add unit tests for Blob mascot component)
7. ⬅️ [7/N] Add unit tests for AboutScreen screen #98 (Add unit tests for AboutScreen screen)
8. ⬅️ [8/N] Add integration tests for HomeScreen screen #99 (Add integration tests for HomeScreen screen)
9. ⬅️ [9/N] Add unit tests for AlgorithmNotFound404 component #100 (Add unit tests for AlgorithmNotFound404 component)
10. ⬅️ [10/N] Add integration tests for AlgoScreen screen #101 (Add integration tests for AlgoScreen screen)
11. ⬅️ [11/N] Add unit tests for ModalSidebarSectionTab component #102 (Add unit tests for ModalSidebarSectionTab component)
12. ⬅️ [12/N] Add unit tests for BigOModal component #103 (Add unit tests for BigOModal component)
13. ⬅️ [13/N] Add unit tests for Pseudocode component #104 (Add unit tests for Pseudocode component)
14. ⬅️ [14/N] Add unit and integration tests for AlgoSection component #105 (Add unit and integration tests for AlgoSection component)
15. ⬅️ [15/N] Add unit and integration tests for root App component #106 (Add unit and integration tests for root App component)
16. ⬅️ [16/N] Add unit tests for animation events and undo functions #107 (Add unit tests for animation events and undo functions)
17. ⬅️ [17/N] Add unit tests for AnimatedObject, AnimatedCircle, and AnimatedHighlightCircle #108 (Add unit tests for AnimatedObject, AnimatedCircle, and AnimatedHighlightCircle)
18. ⬅️ [18/N] Add unit tests for AnimatedRectangle, AnimatedLabel, and AnimatedLine #109 (Add unit tests for AnimatedRectangle, AnimatedLabel, and AnimatedLine)
19. ⬅️ [19/N] Add unit tests for AnimatedLinkedListNode, AnimatedDoublyLinkedListNode, and AnimatedCircularlyLinkedListNode #110 (Add unit tests for AnimatedLinkedListNode, AnimatedDoublyLinkedListNode, and AnimatedCircularlyLinkedListNode)
20. ⬅️ [20/N] Add unit tests for AnimatedBTreeNode and AnimatedSkipListNode #111 (Add unit tests for AnimatedBTreeNode and AnimatedSkipListNode)
21. ⬅️ [21/N] Add unit tests for ObjectManager #112 (Add unit tests for ObjectManager)
22. ⬅️ [22/N] Add unit tests for AnimationMain #113 (Add unit tests for AnimationMain)
23. ⬅️ [23/N] Add unit tests for algo util helpers (PriorityQueue, SkipListHeads, GraphValues) #114 (Add unit tests for algo util helpers (PriorityQueue, SkipListHeads, GraphValues))
24. ⬅️ [24/N] Add unit tests for base Algorithm class and control bar helpers #115 (Add unit tests for base Algorithm class and control bar helpers)
25. ⬅️ [25/N] Add unit tests for ArrayList algorithm #116 (Add unit tests for ArrayList algorithm)
26. 👉 #CURRENT — Add unit tests for LinkedList, DoublyLinkedList, and CircularlyLinkedList algorithms (this PR)
27. ➡️ [27/N] Add unit tests for Stack, Queue, and Deque algorithms (Array and LL variants) #118 (Add unit tests for Stack, Queue, and Deque algorithms (Array and LL variants))

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Addresses #93

Context

Adds comprehensive unit test coverage for the linked list algorithm visualizers:
• LinkedList.js: Singly linked list operations (add/remove head, tail, arbitrary index, traversal).
• DoublyLinkedList.js: Doubly linked list operations (next/prev pointer manipulation, bidirectional traversal, edge cases).
• CircularlyLinkedList.js: Circularly linked list operations (circular pointer maintenance, rotation, single-node and multi-node insertions/removals).

Changes Made

• Introduced src/algo/__tests__/LinkedList.test.js:
• Control registration (addControls, input fields, action buttons, disableUI/enableUI).
• add operations (front, back, middle index) with head/tail updates and size limits.
• remove operations (front, back, middle index) with empty list checks, single element handling, and pointer updates.
• Validation (invalid/out-of-bounds inputs, empty values, capacity constraints).
• reset, clearAll, randomCallback, and URL parameter serialization (setURLData).

• Introduced src/algo/__tests__/DoublyLinkedList.test.js:
• Control registration and UI toggle states.
• Bidirectional add operations (front, back, indexed insertion) with next and prev pointer rewiring.
• remove operations (head, tail, arbitrary index) with empty and single-element edge case checks.
• Pointer integrity assertions, bounds checks, random generation, and clear routines.

• Introduced src/algo/__tests__/CircularlyLinkedList.test.js:
• Circular pointer setup and UI controls initialization.
• add operations (front, back, arbitrary index) verifying circular pointer loops back to head.
• remove operations (front, back, index) asserting circular pointer integrity upon deletion.
• Edge cases: single element removal restoring empty state, full list cap, validation errors, reset, and URL data.

Test Plan

• Ran npm test -- --watchAll=false (all 37 test suites pass, 365 tests passing).
• Ran npm run lint and npm run prettier (0 errors, 0 warnings).