IAV-257/add conversation history and client messages endpoints
- Add getConversationHistory with subagentId, channel, and search filtering
- Add getMessagesByClient for paginated message retrieval by chatId
- Add JPQL query for conversation threads with latest message per client
- Add JPQL query for conversation details by client chatId
- Support createConversationWithoutTicket for agent AI conversations
- Add ConversationThreadDTO and ConversationDetailDTO projections
Closes IAV-257