[Test Improver] Add unit tests for useDragHandler composable

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

Goal and Rationale

shell/components/nav/WindowManager/composables/useDragHandler.ts handles all drag-and-drop tab positioning in the Window Manager — a critical interactive feature. It had zero test coverage. The composable has real branching logic (CENTER guards, shared module-level state, dataTransfer encoding) that benefits from targeted testing.

Approach

• Mocked vuex (useStore) to return a controllable store with state.wm.lockedPositions and state.wm.userPin
• Created a makeDragEvent helper to simulate DragEvent with dataTransfer and preventDefault mocks
• Reset shared module-level refs (dragOverPositionsActive, pinArea) in beforeEach to prevent test pollution
• Verified the shared-state guarantee explicitly (two instances share the same dragOverPositionsActive and pinArea refs)

Tests (31 total)

Group	Tests
initial state	dragOverPositionsActive/dragOverTabBarActive/pinArea defaults
lockedPositions	empty array; populated from store
lockedPosition	no-prop → false; not in list → false; in list → true
pin computed	getter reads userPin; setter commits (non-CENTER) / skips (CENTER)
onDragPositionStart	dataTransfer JSON, dragOverPositionsActive=true, dragOverTabBarActive=true
onDragPositionOver	pinArea update; preventDefault for non-CENTER; no preventDefault for CENTER
onDragPositionEnd	setUserPin commit; switchTab commit (non-CENTER); no switchTab (CENTER); flag reset
onTabBarDragOver	dragOverTabBarActive=true; preventDefault
onTabBarDragLeave	dragOverTabBarActive=false
onTabBarDrop	commit switchTab; reset flag; warn on empty data; no commit on empty data
shared module state	dragOverPositionsActive and pinArea are same ref across instances

Coverage Impact

The composable was previously untested (0% coverage). After this PR it reaches 100% statement/branch/function coverage for useDragHandler.ts.

Test Status

✅ 31/31 tests pass (yarn test:ci --testPathPattern=useDragHandler)
✅ ESLint clean (eslint --max-warnings 0)

Reproducibility

NODE_OPTIONS=--max_old_space_size=8192 yarn test:ci --testPathPattern=useDragHandler

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