Auto-configure ChatSessionRepository bean with event publishing

[jasperblues]

Summary

Currently ChatSessionRepositoryImpl is not created by auto-configuration — consuming apps (e.g., guide) must construct it manually in their @Configuration classes. This means they need to explicitly pass ApplicationEventPublisher (and any future dependencies) to the constructor, which is easy to miss.

Current Behavior

The autoconfigure provides:

• StoredConversationFactory
• SessionEventAwaiter
• InMemoryConversationFactory
• ConversationFactoryProvider

But not ChatSessionRepository. Apps must create it themselves:

@Bean
fun chatSessionRepository(
    @Qualifier("neoGraphObjectManager") graphObjectManager: GraphObjectManager,
    eventPublisher: ApplicationEventPublisher
): ChatSessionRepository {
    return ChatSessionRepositoryImpl(graphObjectManager, eventPublisher)
}

Proposed Behavior

The autoconfigure should provide a default ChatSessionRepository bean (@ConditionalOnMissingBean) that:

• Accepts a GraphObjectManager (or connection properties like spring.neo4j.uri, spring.neo4j.authentication.*)
• Auto-injects ApplicationEventPublisher for SessionCreatedEvent publishing
• Can be overridden by apps that need custom setup

Apps would only need to provide:

1. Connection properties or a compatible GraphObjectManager bean
2. Subtype registrations (e.g., PersistenceManager.registerSubtype) for their user types

Context

This came up while fixing a race condition where StoredConversation.addMessageInternal tried to persist a message before the session existed. The fix (SessionEventAwaiter + SessionCreatedEvent) requires ChatSessionRepositoryImpl to have the eventPublisher — which was missed in the guide's manual construction until caught during review.