Fix session shutdown diagnostics race

[kevinherron]

Summary

Make server shutdown the synchronization point for session listener quiescence so diagnostics and namespace teardown no longer race with session callbacks.

• Stop accepting new sessions as soon as shutdown begins so new CreateSession work cannot enqueue diagnostics callbacks during teardown.
• Drain in-flight session listener callbacks before namespace and diagnostics shutdown, while still avoiding self-deadlock when shutdown is requested from a listener callback.
• Make session close idempotent so concurrent shutdown, timeout, and close paths notify lifecycle listeners only once.
• Shut down managed lifecycle children before unregistering their node managers, keeping diagnostics nodes valid during child teardown.
• Remove sleep-based shutdown ordering from examples and integration test cleanup.

Key Changes

• Server shutdown ownership: OpcUaServer.shutdown() now uses a shared shutdown future so concurrent callers observe the same teardown instead of running namespace and diagnostics shutdown multiple times.
• Session listener quiescence: SessionManager tracks shutdown state, rejects new sessions with Bad_Shutdown, drains listener work, and skips queued callbacks once shutdown starts.
• Re-entrant shutdown handling: Session listener dispatch marks callback threads so shutdown requested from inside a listener does not wait on itself, while external callers still wait for the active shutdown to finish.
• Lifecycle ordering: Managed namespace/address-space lifecycle managers now shut down in inverse registration order so child diagnostics and subscription lifecycles run before node managers are unregistered.
• Test cleanup: Sleep-based waits were removed because server.shutdown() now provides the ordering guarantee.

Testing

• SessionTest covers idempotent Session.close(...) behavior and verifies lifecycle listeners are notified once.
• ServerShutdownSessionDiagnosticsRaceTest covers blocked session listener shutdown, concurrent shutdown callers, and shutdown requested from inside a session listener callback.
• mvn -q spotless:apply
• mvn -q -pl opc-ua-sdk/sdk-server -am test -Dtest=SessionTest -Dsurefire.failIfNoSpecifiedTests=false
• mvn -q -pl opc-ua-sdk/integration-tests -am test -Dtest=ServerShutdownSessionDiagnosticsRaceTest -Dsurefire.failIfNoSpecifiedTests=false
• mvn -q clean compile