fix: fail loudly on an unhandled hub lifetime result

AddLiveControlConnection returns Union3<HubLifetime, NotFound, Busy> and all
three are handled, but a switch statement is not exhaustiveness-checked. If the
union ever widens, the unhandled case would fall through to reporting
LiveControlOutcome.Connected and returning Success with a null _hubLifetime,
surfacing only later as "Hub lifetime is null but was accessed". Matches the
UnreachableException default already used by the switch expressions below.

Author: hhvrc

LiveControlGateway/Controllers/LiveControlController.cs

@@ -238,6 +238,10 @@ public async Task UpdatePermissions(OpenShockContext db)
             case LifetimeManager.HubLifetime hubLifetime:
                 _hubLifetime = hubLifetime;
                 break;
+            // A switch statement is not exhaustiveness-checked, so without this an unhandled case would
+            // fall through to reporting Connected with a null lifetime and only fail later on first access.
+            default:
+                throw new UnreachableException();
         }
 
         _metrics.LiveControlAttempt(GatewayMetrics.LiveControlOutcome.Connected);