[AI assisted] MM-63304: Wrap around dqPtr while unmarshalling (#30371)

If the deadQueue was full, we would be incorrectly sending
the wrong index for dqPtr. Fixed that and added a test case.

https://mattermost.atlassian.net/browse/MM-63304

```release-note
NONE
```

server/channels/app/platform/websocket_reliable.go

@@ -156,7 +156,7 @@ func (ps *PlatformService) UnmarshalDQ(buf []json.RawMessage) ([]*model.WebSocke
 		// Same as active queue, this can never be out of bounds because all dead queues
 		// are of deadQueueSize.
 		dq[dqPtr] = item
-		dqPtr++
+		dqPtr = (dqPtr + 1) % deadQueueSize
 	}
 	return dq, dqPtr, nil
 }