fix: narrow possibly undefined system in beszel test

TypeScript reports 'system' is possibly undefined since it comes from
array indexing. Replace expect().toBeDefined() with an explicit throw
guard so the type narrows for subsequent property accesses.

packages/integrations/src/beszel/test/beszel-integration.spec.ts

@@ -75,8 +75,10 @@ describe.skipIf(!shouldRun)("BeszelIntegration (real instance)", () => {
     test("each system has required fields", async () => {
       const systems = await integration.getSystemsAsync();
       const system = systems[0];
-      expect(system).toBeDefined();
-      systemId = system?.id ?? "";
+      if (!system) {
+        throw new Error("Expected at least one system");
+      }
+      systemId = system.id;
 
       expect(system.id).toBeDefined();
       expect(system.name).toBeDefined();