Simplify app event watcher test polling

[alfonso-noriega]

WHY are these changes introduced?

No issue.

The app event watcher tests had a small custom polling loop that duplicated Vitest's wait helper and used a manual setTimeout/Date.now timeout path. Using the framework helper keeps the test wait behavior simpler and preserves useful timeout diagnostics.

WHAT is this pull request doing?

• Replaces the custom waitForWatcherEmit polling promise with vi.waitFor.
• Keeps the same 10ms polling interval and timeout parameter.
• Includes the emitted event names in the assertion message when the expected event is not observed.

How to test your changes?

• pnpm --filter @shopify/app vitest src/cli/services/dev/app-events/app-event-watcher.test.ts ✅
• pnpm --filter @shopify/app lint ✅
• git diff --check ✅
• pnpm --filter @shopify/app type-check ⚠️ attempted; fails on current origin/main due existing @shopify/organizations/Organization type resolution errors outside this diff.

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
• The change is not user-facing; no changeset is needed.