[21/N] Add unit tests for ObjectManager

[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. 👉 #CURRENT — Add unit tests for ObjectManager (this PR)
22. ➡️ [22/N] Add unit tests for AnimationMain #113 (Add unit tests for AnimationMain)

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Addresses #93

Context

Adds unit test coverage for ObjectManager.js, the central scene graph manager responsible for object registry, layer routing, edge tracking, cardinal alignment, and canvas render ordering.

Changes Made

• Introduced src/anim/__tests__/ObjectManager.test.js covering:
• Instantiation and registration across all 9 visual entity types (Circle, HighlightCircle, Rectangle, Label, LinkedListNode, DoublyLinkedListNode, CircularlyLinkedListNode, SkipListNode, BTreeNode) with duplicate ID detection.
• Node property modification and retrieval (coordinates, dimensions, text labels, colors, alpha, alwaysOnTop, and edge thicknesses).
• Edge connection management (forward and back edges, customizable color/thickness/curve, edge property updates, disconnection undo creation, and incident deletion).
• Layer management (activation, toggling, bulk setting, and recursive visibility propagation to connected edges).
• Alignment algorithms (alignTop, alignBottom, alignLeft, alignRight, and relative coordinate helpers).
• The full canvas draw pipeline across unhighlighted, highlighted, alwaysOnTop nodes, pulsing edges, and status report.

Test Plan

• Ran npm test -- --watchAll=false
• Ran npm run lint and npm run prettier
• Verified 100% statement, function, and line coverage across ObjectManager.js