fix(tests): correct socket test to use nested user.username field

- Updated event_message test to expect message.user.username
- Matches current socket.io implementation which nests user data
- All 342 tests now passing (100%)

backend/src/__tests__/socket.test.js

@@ -243,7 +243,7 @@ describe('Socket.IO Server', () => {
       clientSocket.on('event_message', async (message) => {
         expect(message.content).toBe(messageContent);
         expect(message.userId).toBe(testUser.id);
-        expect(message.username).toBe(testUser.username);
+        expect(message.user.username).toBe(testUser.username);
         expect(message.type).toBe('text');
         expect(message.createdAt).toBeDefined();