test: improve unit tests with new mocks and WebSocket validation

- Added `markAsRead` and `countUnreadMessages` mocks to `MessagesService` tests.
- Included enriched comment retrieval and WebSocket notification validation in `CommentsService` tests.
- Updated dependency injection to include `EventsGateway` in `CommentsService` tests.

backend/src/messages/messages.service.spec.ts

@@ -17,6 +17,8 @@ describe("MessagesService", () => {
 		findAllConversations: jest.fn(),
 		isParticipant: jest.fn(),
 		findMessagesByConversationId: jest.fn(),
+		markAsRead: jest.fn(),
+		countUnreadMessages: jest.fn(),
 	};
 
 	const mockEventsGateway = {