feat: Enhance agent selection and conversation handling

- Added functionality to select agents in the Navigation component, allowing users to reset conversations and set the selected agent.
- Updated the MessageInput component to conditionally show source and tool buttons based on the selected agent.
- Modified the Conversation component to handle agent-specific queries and manage file uploads.
- Improved conversation fetching logic to include agent IDs and handle attachments.
- Introduced new types for conversation summaries and results to streamline API responses.
- Refactored Redux slices to manage selected agent state and improve overall state management.
- Enhanced error handling and loading states across components for better user experience.

frontend/src/store.ts

@@ -40,6 +40,7 @@ const preloadedState: { preference: Preference } = {
     ],
     modalState: 'INACTIVE',
     paginatedDocuments: null,
+    selectedAgent: null,
   },
 };
 const store = configureStore({