Improve integration test timing: replace fixed sleeps with event-driven waiting or fake timers

[doughgle]

The integration tests in expandAll-integration.test.ts and roundTrip-expand-collapse.test.ts use fixed setTimeout delays (1000ms, 500ms, 200ms) to wait for tree view operations. This makes tests slow and brittle. Consider:

• Polling with a timeout instead of fixed sleeps (pattern already partially used in roundTrip-expand-collapse.test.ts lines 58-63)
• Exposing events/promises from the extension that tests can await
• A fake timers library (e.g. sinon) to control timing in unit tests

Originally posted by @Copilot in #12 (comment)