Add test coverage for allocation assignment endpoint

app/Http/Controllers/Api/Client/Servers/NetworkAllocationController.php

@@ -122,6 +122,12 @@ class NetworkAllocationController extends ClientApiController
      */
     public function store(NewAllocationRequest $request, Server $server): array
     {
+        if ($server->allocations()->count() >= $server->allocation_limit) {
+            throw new DisplayException(
+                'Cannot assign additional allocations to this server: limit has been reached.'
+            );
+        }
+
         $allocation = $this->assignableAllocationService->handle($server);
 
         return $this->fractal->item($allocation)