Prevent duplicate event listener registrations

[kevinherron]

Motivation

• Fix a regression where the server EventNotifier allowed duplicate EventListener registrations, enabling a remote client to amplify event delivery work and cause unbounded listener growth (DoS risk).

Description

• Replace the ServerEventNotifier backing collection from a synchronized List to a synchronized LinkedHashSet to make register() idempotent and prevent duplicate listener entries while preserving insertion order and snapshot-based delivery (List.copyOf(eventListeners)).
• Add import for LinkedHashSet and change the field type to Set<EventListener> in opc-ua-sdk/sdk-server/src/main/java/org/eclipse/milo/opcua/sdk/server/OpcUaServer.java.
• Add regression tests in opc-ua-sdk/sdk-server/src/test/java/org/eclipse/milo/opcua/sdk/server/ServerEventNotifierTest.java that assert repeated register() calls result in a single delivery and that a single unregister() removes the listener.

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Codex Task