Add concurrency tests for ApplicationManager thread safety

[sfloess]

Part of #423: Race condition potential in ApplicationManager concurrent operations

Problem

The race conditions described in #423 need targeted concurrency tests to verify the fixes and prevent regressions. Without tests, future maintenance could reintroduce lock ordering bugs or iteration safety issues.

Tasks

• Write a test for concurrent deploy + shutdown (the specific race described in ISSUE-10: Race condition potential in ApplicationManager concurrent operations #423)
• Write a test for concurrent deploy + undeploy on the same application
• Write a stress test with multiple threads doing deploy/undeploy/shutdown simultaneously
• Write a test verifying listener notifications are not lost under concurrent operations
• Use CountDownLatch or CyclicBarrier to force thread interleaving at critical points
• Consider using jcstress or Thread Weaver for systematic concurrency testing

Acceptance Criteria

• Tests reliably detect the race conditions described in ISSUE-10: Race condition potential in ApplicationManager concurrent operations #423 (fail before fix, pass after)
• Tests cover: deploy+shutdown race, deploy+undeploy race, listener notification completeness
• Tests are not flaky (use synchronization primitives, not sleep-based timing)