✨(backend) notify participants only if the room exists

Improves sendData reliability by preventing execution when the room
doesn’t exist.

This change addresses errors in staging and production where waiting
participants arrive before the room owner creates the room.

In remote environments, the LiveKit Python SDK doesn’t return a clean
Twirp error when the room is missing; instead of a proper "server unknown"
response, it raises a ContentTypeError, as if the LiveKit server weren’t
responding with a JSON payload, even though the code specifies otherwise.

While the issue cannot be reproduced locally,
this should help mitigate production errors.

Part of a broader effort to enhance data transmission reliability.

Importantly, a participant requesting entry to a room before the owner
arrives should not be considered an exception.

src/backend/core/services/lobby.py

@@ -12,7 +12,12 @@ from django.conf import settings
 from django.core.cache import cache
 
 from asgiref.sync import async_to_sync
-from livekit.api import LiveKitAPI, SendDataRequest, TwirpError  # pylint: disable=E0611
+from livekit.api import (  # pylint: disable=E0611
+    ListRoomsRequest,
+    LiveKitAPI,
+    SendDataRequest,
+    TwirpError,
+)
 
 from core import models, utils
 
@@ -343,7 +348,18 @@ class LobbyService:
         }
 
         lkapi = LiveKitAPI(**settings.LIVEKIT_CONFIGURATION)
+
         try:
+            room_response = await lkapi.room.list_rooms(
+                ListRoomsRequest(
+                    names=[str(room_id)],
+                )
+            )
+
+            # Check if the room exists
+            if not room_response.rooms:
+                return
+
             await lkapi.room.send_data(
                 SendDataRequest(
                     room=str(room_id),