🐛(backend) fix info panel crash for unregistered rooms

The info panel was crashing when opening a room that was not
registered in the database (with ALLOW_UNREGISTERED_ROOMS=true),
because the API response did not include the room slug.

Instead of adding frontend fallbacks, update the unregistered-room
response to include the slug, keeping the API contract consistent
with registered rooms.

Note: this still relies on the unregistered-room response staying
aligned with the registered-room schema. Any future divergence
between the two responses could introduce similar issues.
A refactoring on the backend side is needed.

It closes #1441.

src/backend/core/tests/rooms/test_api_rooms_retrieve.py

@@ -130,6 +130,9 @@ def test_api_rooms_retrieve_anonymous_unregistered_allowed(mock_token):
     assert response.status_code == 200
     assert response.json() == {
         "id": None,
+        "slug": "unregistered-room",
+        "access_level": "public",
+        "is_administrable": False,
         "livekit": {
             "url": "test_url_value",
             "room": "unregistered-room",
@@ -162,6 +165,9 @@ def test_api_rooms_retrieve_anonymous_unregistered_allowed_not_normalized(mock_t
     assert response.status_code == 200
     assert response.json() == {
         "id": None,
+        "slug": "reunion",
+        "access_level": "public",
+        "is_administrable": False,
         "livekit": {
             "url": "test_url_value",
             "room": "reunion",