[12/N] Add unit tests for BigOModal component

[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 (this PR)
13. ➡️ [13/N] Add unit tests for Pseudocode component #104 (Add unit tests for Pseudocode component)

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Addresses #93

Context

Adds unit test coverage for BigOModal.js to ensure time complexity tables, equation formatting, individual cell blur toggling, and global reveal/hide controls operate reliably.

Changes Made

• Introduced src/modals/__tests__/BigOModal.test.js to unit test BigOModal.

Test Plan

• Ran npm test -- --watchAll=false
• Ran npm run lint and npx prettier --check src/
• Verified that the CI checks pass