test: Improve coverage for MultiSelectFilter component

[ParasKhandelwal1616]

Description

Improved test coverage for the MultiSelectFilter component located in
frontend/src/components/ui/multi-select.tsx. Comprehensive unit tests were added to verify rendering, user
interactions, and state management, achieving 100% test coverage for this component.

• Fixes: Add test suites for Frontend files to bump overall coverage #352

Checklist

• Ran npx prettier --write . (for formatting)
• Ran npm test (for JS/TS testing)
• Added unit tests, if applicable
• Verified all tests pass
• Updated documentation, if needed (N/A)

Additional Notes

The new test suite in frontend/src/components/ui/tests/multi-select-filter.test.tsx includes:

• Rendering Tests: Verified component renders correctly with titles, placeholders, custom icons, and varied
  className props.
• Interaction Tests:
  • Opening and closing the popover using clicks and the Escape key.
  • Selecting and deselecting multiple options.
  • Clearing all selections via the "All" toggle.
• Search Logic: Validated case-insensitive filtering and the "No results found" state.
• Completion Stats: Confirmed that task completion metrics (count and percentage) display correctly when
  provided.

Coverage Results:

• Statements: 100%
• Branches: 100%
• Functions: 100%
• Lines: 100%
  hots, or context -->

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