InMemoryStorableConversation should store from/to attribution

[jasperblues]

Problem

InMemoryStorableConversation.addMessageFromTo() silently ignores the from and to parameters, delegating directly to addMessage(). This means message attribution is lost in-memory, which could cause subtle bugs when testing multi-party chat scenarios.

Current behavior

override fun addMessageFromTo(message: Message, from: User?, to: User?): Message {
    return addMessage(message)
}

Proposed fix

Store attribution in a side map (e.g., Map<String, Pair<User?, User?>> keyed by message ID) so that in-memory conversations faithfully track from/to like StoredConversation does with Neo4j graph relationships.

This would make in-memory conversations a more accurate stand-in for stored conversations in tests.

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