IAV-1266/feat(customer-care): add API to update conversation title
- Add AgentConversationPaths.Conversations.UPDATE_TITLE and PUT endpoint on AgentConversationController.
- Add UpdateConversationTitleRequest (conversationUid, title) with validation.
- Implement AgentConversationService.updateConversationTitle: load session by uid, trim title, persist, return mapped response; map not-found / update failures.