IAV-1337/feat(Conversations): add ticket unassign endpoint and enrich conversation detail mapping
- Add DELETE /tickets/{ticketUid}/assign with TicketService.unassignTicket to clear collaborator assignment and notify previous assignee.
- Extend ConversationDetailDTO with senderName and include HUMAN_AGENT in conversation detail queries.
- Refactor ConversationRepository chat lookups to be ticket-aware and return consistent sender metadata for both ticket-linked and legacy rows.
- Fix chat/client query method alignment in conversation history retrieval.