[Bug] Async shutdown is not idempotent and empty/timed waits fail #11252

Before Creating the Bug Report

• I inspected the current develop code and searched related issues/open pull requests.

Summary

AsyncShutdownHelper.shutdown() only checks a flag that is set by await(), so repeated shutdown calls can launch every target again and replace the completion latch. An empty target list leaves the latch null, and a timed-out await permanently prevents later awaits.

Steps to Reproduce

Call new AsyncShutdownHelper().shutdown().await(...), or time out an await while a target is blocked, release the target, and await again.

Expected Behavior

Publish one latch when shutdown starts, synchronize target registration/start, and let all await calls observe the same completion state.

Environment / Additional Context

• Reproduced against develop commit 78b96bc5e21216cd7896efae08f90c5cde4cae53.
• Regression test: AsyncShutdownHelperLifecycleTest.
• The regression fails before the fix and passes afterward on JDK 8.
• Proposed change is limited to the existing implementation; no protocol or new API is introduced.