Fix race condition in CleanupAddon.subscribeRenderingChanged

[vogella]

The test ensureCleanUpAddonCleansUp was failing intermittently because CleanupAddon deferred container cleanup via Display.asyncExec(), creating a race condition where event loop pumping during part activation could interfere with async cleanup tasks.

Root cause analysis:

• When hidePart() is called on parts with REMOVE_ON_HIDE_TAG, it triggers synchronous EMF model change events via UIEventPublisher
• Event delivery is fully synchronous: EventBroker.send() ->
  EventAdmin.sendEvent() -> Display.syncExec() -> handler execution
• The subscribeRenderingChanged handler detected visCount==0 but deferred cleanup via asyncExec (line 352)
• Meanwhile, activate() calls during hidePart() pumped the event loop (via focusGui/SWT focus handling), potentially consuming queued asyncExecs before they could observe the final state
• This created timing-dependent behavior where cleanup could be skipped

Fix:

• Remove asyncExec wrapper from subscribeRenderingChanged's visCount==0 path
• Since event delivery is synchronous, visCount already reflects the actual state at event time - no need to defer and re-check
• Container is now hidden immediately when last renderable child becomes non-renderable, before any event loop pumping can interfere
• Update test to use simple spinEventLoop + assertFalse instead of 30-second waitForCondition, as cleanup is now synchronous

This fixes the flaky test that failed ~1-3% of the time even with the 30-second timeout. The subscribeTopicChildren handler still uses asyncExec for children removal/sash unwrapping, but toBeRendered cleanup is now deterministic.

Fixes #3581

[github-actions]

Test Results

 3 024 files  ±0   3 024 suites  ±0   2h 10m 19s ⏱️ - 12m 17s
 8 232 tests ±0   7 984 ✅ ±0  248 💤 ±0  0 ❌ ±0 
23 520 runs  ±0  22 729 ✅ ±0  791 💤 ±0  0 ❌ ±0 

Results for commit aa9e3b7. ± Comparison against base commit b44e9ab.

[vogella]

AFAICS this is a save change (famous last words) and should finally fix our forever flaky cleanup race condition revealed in the cleanup test.

[vogella]

I suggest to wait for the beginn of the next release developement for this one.