fix: use CopyOnWriteArrayList for thread-safe conversation history in InMemoryPersistenceFacade

[Copilot]

ConcurrentHashMap does not protect the values it stores — mutating a plain ArrayList retrieved via computeIfAbsent is not thread-safe and can cause lost updates or ConcurrentModificationException under concurrent load.

Changes

• InMemoryPersistenceFacade — replace new ArrayList<>() with new CopyOnWriteArrayList<>() for the conversations map values, making concurrent reads (loadConversation, listConversationIds, latestTimestamp) and writes (appendEvent) safe without external synchronization

// before
conversations.computeIfAbsent(event.conversationId(), key -> new ArrayList<>()).add(event);

// after
conversations.computeIfAbsent(event.conversationId(), key -> new CopyOnWriteArrayList<>()).add(event);

Note: the originally flagged class (AgentA2ATaskRepository) was removed in PR #1 and replaced by AgentA2ATaskAdapter, which delegates state management to the thread-safe A2ATaskService. This change addresses the same pattern in the surviving in-memory persistence layer.

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[Copilot]

Pull request overview

This PR adjusts the in-memory persistence implementation to use a thread-safe list type for storing per-conversation events, aligning InMemoryPersistenceFacade with concurrent access patterns in the agent runtime (including fallback-to-in-memory scenarios).

Changes:

• Replaced ArrayList with CopyOnWriteArrayList for conversations event storage in appendEvent().

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