[Test Improver] Add unit tests for useInterval composable

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Goal and Rationale

shell/composables/useInterval.ts had no test coverage. It registers a setInterval on onMounted and clears it on onBeforeUnmount — a lifecycle-driven side effect that is easy to misuse (e.g. timer leaks, interval not cleared on unmount). Testing this verifies correct lifecycle integration and that the cleanup guard works properly.

Approach

Each test mounts/unmounts a minimal defineComponent harness that calls useInterval, then advances fake timers to observe call counts. This isolates the composable without requiring a real component.

Tests (8 total):

#	Test	What it verifies
1	does not call fn before the component is mounted	fn is only invoked via setInterval, not eagerly
2	calls fn after the specified delay once mounted	interval starts on onMounted
3	calls fn repeatedly on each interval tick	setInterval (not setTimeout) behavior
4	does not call fn before the delay elapses	off-by-one boundary at delay - 1 ms
5	clears the interval on unmount so fn is not called afterwards	clearInterval guard in onBeforeUnmount
6	stops calling fn immediately when unmounted mid-interval	cleanup works even partway through a tick
7–8	table-driven: short (100 ms / 350 ms → 3 calls) and long (2000 ms / 5000 ms → 2 calls) delays	delay parameter is respected accurately

Coverage Impact

useInterval.ts moves from 0 % → ~100 % statement/branch coverage (all branches: mounted callback, unmount guard, if (interval.value) check).

Trade-offs

• Uses jest.useFakeTimers() per-test to avoid wall-clock delays.
• defineComponent harness keeps tests self-contained and avoids mocking Vue internals.

Reproducibility

YARN_IGNORE_ENGINES=true NODE_OPTIONS=--max_old_space_size=8192 yarn test:ci --testPathPattern=shell/composables/useInterval

Test Status

✅ All 8 tests pass. ESLint reports 0 warnings/errors.

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