Validate connection data integrity in ConnectionManager

Fix audit warnings about empty string defaults hiding data corruption:

1. get_connection_info(): Validate required fields (user_id, connected_at,
   last_seen) exist before creating ConnectionInfo. Return None and log
   warning for corrupted records instead of returning invalid data.

2. unregister_connection(): Log warning if user_id is missing during
   cleanup. Existing code safely handles this (skips cleanup), but now
   we have visibility into data corruption.

Test added for corrupted data case.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/connection_manager.py

@@ -230,6 +230,10 @@ class ConnectionManager:
             user_id = data.get("user_id", "")
             game_id = data.get("game_id", "")
 
+            # Warn if user_id is missing - indicates corrupted data
+            if not user_id:
+                logger.warning(f"Connection {sid} has no user_id - data may be corrupted")
+
             # Remove from game connection set if in a game
             if game_id:
                 game_conns_key = self._game_conns_key(game_id)
@@ -392,12 +396,25 @@ class ConnectionManager:
             if not data:
                 return None
 
+            # Validate required fields - if missing, data is corrupted
+            user_id = data.get("user_id")
+            connected_at = data.get("connected_at")
+            last_seen = data.get("last_seen")
+
+            if not user_id or not connected_at or not last_seen:
+                logger.warning(
+                    f"Corrupted connection record for {sid}: "
+                    f"missing required fields (user_id={bool(user_id)}, "
+                    f"connected_at={bool(connected_at)}, last_seen={bool(last_seen)})"
+                )
+                return None
+
             return ConnectionInfo(
                 sid=sid,
-                user_id=data.get("user_id", ""),
+                user_id=user_id,
                 game_id=data.get("game_id") or None,
-                connected_at=datetime.fromisoformat(data["connected_at"]),
-                last_seen=datetime.fromisoformat(data["last_seen"]),
+                connected_at=datetime.fromisoformat(connected_at),
+                last_seen=datetime.fromisoformat(last_seen),
             )
 
     async def get_user_connection(self, user_id: str | UUID) -> ConnectionInfo | None:

backend/tests/unit/services/test_connection_manager.py

@@ -462,6 +462,30 @@ class TestQueryMethods:
 
         assert result is None
 
+    @pytest.mark.asyncio
+    async def test_get_connection_returns_none_for_corrupted_data(
+        self,
+        manager: ConnectionManager,
+        mock_redis: AsyncMock,
+    ) -> None:
+        """Test that get_connection returns None for corrupted records.
+
+        If a connection record is missing required fields (user_id,
+        connected_at, last_seen), it indicates data corruption. The
+        method should return None and log a warning rather than
+        returning a ConnectionInfo with invalid/empty data.
+        """
+        # Record missing user_id
+        mock_redis.hgetall.return_value = {
+            "game_id": "game-123",
+            "connected_at": "2024-01-01T00:00:00+00:00",
+            "last_seen": "2024-01-01T00:00:00+00:00",
+        }
+
+        result = await manager.get_connection("corrupted-sid")
+
+        assert result is None
+
     @pytest.mark.asyncio
     async def test_is_user_online_returns_true_for_connected_user(
         self,