Make metadata timer tests deterministic

[alfonso-noriega]

WHY are these changes introduced?

Fixes #0000

The metadata timer tests used short real sleeps to create measurable durations. Those sleeps make the tests depend on scheduler timing even though the behavior under test only needs elapsed performance.now() values.

WHAT is this pull request doing?

Replaces the real sleeps in the nested metadata timer tests with a tiny mocked performance clock. The tests now advance the clock explicitly while keeping the same nested timer structure and performance measure assertions.

No changeset is included because this is test-only maintenance with no user-facing behavior change.

How to test your changes?

• pnpm --filter @shopify/cli-kit vitest -- src/public/node/metadata.test.ts
• pnpm --filter @shopify/cli-kit type-check
• pnpm --filter @shopify/cli-kit lint
• git diff --check

Post-release steps

None.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• This change is not user-facing, so no changeset is required