style(tests): narrow promptResidual instead of non-null assertion

Author: TheGreatAxios

src/agent/model-family-policy.test.ts

@@ -111,7 +111,8 @@ describe("resolveModelFamilyPolicy", () => {
       });
       expect(leaf.family).toBe("grok");
       expect(leaf.promptResidual).toBeDefined();
-      expect(leaf.promptResidual!.split("\n")).toHaveLength(4);
+      if (!leaf.promptResidual) throw new Error("expected promptResidual to be defined");
+      expect(leaf.promptResidual.split("\n")).toHaveLength(4);
       expect(leaf.promptResidual).toContain("Tool budget:");
     });