[Test Improver] Add unit tests for getFirstFocusableElement in focusTrap composable

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🤖 This PR was created by Test Improver, an automated AI assistant focused on improving test coverage.

Goal and Rationale

Add unit tests for getFirstFocusableElement in shell/composables/focusTrap.ts. This function is responsible for finding the first keyboard-focusable element within a container — it is used by useBasicSetupFocusTrap and useWatcherBasedSetupFocusTrapWithDestroyIncluded to anchor the focus trap. It had zero test coverage despite containing non-trivial logic:

• A multi-type CSS selector covering 7 element categories
• Explicit exclusion of tabindex="-1" elements
• Filtering out disabled elements from the result set
• Fallback to document.body when no candidates remain

Approach

• Tested getFirstFocusableElement in isolation — it is a pure function with no Vue lifecycle hooks or Vuex dependencies, so no component mount is required.
• Used JSDOM's document.createElement to build controlled containers for each case.
• Used it.each with named-object entries (desc field) for the element-type cases to avoid repetition.
• Covered both the happy path (each focusable type found) and the unhappy paths (all disabled, tabindex=-1, empty container).

Coverage Impact

getFirstFocusableElement goes from 0% to 100% statement/branch/function coverage.

Metric	Before	After
getFirstFocusableElement statements	0%	100%
getFirstFocusableElement branches	0%	100%

Test Cases (16 total)

• returns document.body when the container has no focusable elements
• returns document.body when no element argument is provided and document has no focusable elements
• returns the button element (via it.each over all 8 focusable types)
• skips a disabled button and returns the next non-disabled focusable element
• skips a disabled input and returns the next non-disabled focusable element
• returns document.body when every focusable element is disabled
• does not return an element with tabindex="-1"
• returns the first of multiple focusable elements
• returns the first non-disabled element when leading elements are all disabled

Trade-offs

• Tests use JSDOM DOM APIs (document.createElement, innerHTML) — low maintenance burden.
• useBasicSetupFocusTrap and useWatcherBasedSetupFocusTrapWithDestroyIncluded are not tested here; they rely on the focus-trap library and Vue lifecycle hooks and are best tested via component integration tests.

Reproducibility

NODE_OPTIONS=--max_old_space_size=8192 yarn test:ci --testPathPattern=shell/composables/focusTrap

Test Status

✅ All 16 new tests pass
✅ All 42 existing composable tests continue to pass
✅ ESLint: no warnings or errors

Warning

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